Most butter contains about 15 to 17 percent water. When the pie dough goes into the oven, the water turns to steam, which is what helps create layers in your dough. In other words, it is the sheets of butter that make your pie dough flaky. If your butter is somewhat warm, then you end up with something that is more like cookie dough than flaky pie dough. Not the end of the world by any means—and a tender, crumbly pie dough is still a good dough—but for a pie crust that flakes and shatters and impresses with its many layers, keep your butter cold, cold, cold.
When do you want your ingredients to be at room temperature? There are two good examples of this. The first is when you are combining sugar and butter for a cake or cookie. If you look at sugar under a microscope you see why they are called sugar crystals. They have jagged edges, and when you mix sugar into room temperature butter, these edges act as an army of little workers with shovels carving out miniscule air pockets within the butter. If your butter is too cold, the sugar—try as it might—can't dig its way through the hard chilled butter; if the butter is too warm, the sugar merely sloshes around, not really being effective at all.
If it is at room temperature, however, that sugar can work its magic and aerate the butter. The act of combining butter and sugar together in this way is actually called "creaming" because when done properly the butter turns light and white like cream. Once you've created a multitude of air pockets, the baking powder or soda you add to the cake/cookie later on expands these air pockets and you end up with a light, tender, fluffy pastry. And all because you started with room temperature butter!
A second example of when ingredients should be at room temp is when you add a liquid such as milk or buttermilk or eggs into a cookie or cake batter. Imagine the butter and sugar you've just creamed together: an aerated fluffy room-temperature glorious mass. The next step in the recipe calls for adding eggs or other liquid to the butter-sugar. If your eggs/liquid are cold and not at the same temperature as what you are about to mix them into, the butter will immediately harden into little cold pellets. And when you bake your cake/cookie you'll end up with lots of little holey pockets from the butter bits. Not good! To keep your crumb even and soft, make sure your ingredients are at the same temperature when combining them, ensuring seamless emulsification.
Moving on to warmer temperatures, there's a valuable baking technique called tempering that will prevent you from making scrambled eggs when you are trying to make a pastry cream or ice cream base. First you start with something like milk or cream that you heat up in a saucepan until it just about comes to a boil. In a separate bowl you mix eggs and sugar. Now you want to combine the hot liquid with the egg-sugar mixture. Do you just mix the two together? Noooooooooo. That's like jumping into the pool, cannonball style. In pastry you want to dip your toe in, one toe at a time, and slowly ease your way in.
Thus we have tempering. Take a little bit of the hot mixture and whisk it slowly into the cold mixture. Keep adding a bit of hot to the cold until the cold is no longer cold. Once about half of the hot mixture is combined with the cold you can easily combine the rest of the ingredients together without fear of making sweetened scrambled eggs.
https://www.theatlantic.com/health/archive/2011/01/baking-for-beginners-an-introduction-to-temperature/69555/
Butterfat and Moisture Content in Butter
Butter is made by churning cream until it separates into liquids (buttermilk) and solids (butterfat). In North America, commercially sold butter must contain at least 80 percent butterfat, while European butters typically contain at least 82 percent and up to about 85 percent. The remaining contents of the butter's makeup includes water and milk solids. The lower the butterfat content, the more liquid (and less fat) is being added to your recipe; the higher the butterfat content, the more fat (and less liquid).
So what does this all mean when it comes to baking? Since having less butterfat essentially means more liquid, then consider what adding a bit more liquid will do to your doughs and batters. This small increase might not be as noticeable in simple brownies or quick-breads, but the additional liquid can weigh down doughs and leave pastries a bit tougher. Butter with more butterfat will also remain solid slightly longer in the oven. Want crisper, flakier croissants and puff pastry? Go for the butter with the higher butterfat content.
Also, it is worth noting the different between the moisture and moistness butter adds to a recipes. Moisture refers to the liquid content while moistness refers to the fat. Nearly all recipes call for both, but for different reasons. In general, fat (including that in butter) tenderizes baked goods. Liquids (again, including that in butter) help hydrate proteins (found in flours and starches), bind ingredients together, and also aid in moistening.
Butter in Recipes
Moisture and flavor might be the most obvious reasons to use butter in baked goods, but there are several other roles butter is playing as well. In things like cakes, cookies, and muffins, butter coats the proteins and starches during the mixing step and results in a more delicate crumb.
In many of these types of pastries, the butter is creamed with the sugar before being mixed with other ingredients. Through this process, the sugar granules actually cut into the softened butter and air is forced into the mixture, which ultimately helps to leaven the pastry.
Even in recipes that do not call for the creaming method, butter assists in leavening by creating steam when placed in a hot oven. Recall what makes up butter other than butterfat? The liquid portion of butter adds moisture (as opposed to the moistness added from the fat), and in baked goods like puff pastry and croissants, the liquid in the butter begins to evaporate and create steam, which lifts the pastry as it bakes.
Why Butter Temperature Matters
Temperature is a key factor in how butter behaves within a recipe and how it mixes with other ingredients in a batter or dough. Rarely does a recipe list butter without noting if it should be cold from the fridge, softened to room temperature, or even melted.
With softened butter, the fat can be easily creamed together with sugar, or used to coat flour particles. This creates a more even distribution of fat throughout the dough or batter, yielding a tender final product. Like using room-temperature eggs, room-temperature butter creates a more homogenous batter and prevents buttercream from "breaking" (more on this tomorrow).
Softened butter should still be cool, but malleable. It should be able to hold its shape and still firm enough that if you press your finger into it, the impression is clean. It should not be squishy, oily, or appear melted. Too-warm or melted butter loses its ability to cream and hold air when beaten.
In most kitchens, it will take about 30 to 60 minutes to soften butter to room temperature. Forgot to remove your butter from the fridge? Need to speed things up? Try cutting the butter into smaller pieces, carefully grate it with a box grater, pound it flat with a rolling pin, or use the indirect heat of a double-boiler (making sure it does not melt!).
Very cold butter is used in recipes where you don't want the butter to combine with the rest of the ingredients; you want it to stay cohesive. Pie dough, puff pastry, biscuits, and scones all typically call for very cold butter so it remains intact and unincorporated, which leads to distinct layers in the finished baked goods.
Why layers? As mentioned earlier, butter creates steam as it melts in a hot oven, and in pastries where cold butter is used, the steam from the melting butter expands between the layers of dough. This creates pockets of air, yielding a flaky end product. (Quick tip: Try grating frozen butter into biscuit or pie dough in order to easily distribute the butter without over-working it, and in turn, softening and warming it!)
On occasion a recipe will call for melted butter. Here, the butter provides moistness and flavor but does not contribute to structure. Since it is not being creamed and aerated nor kept in cold pieces that create steam in the oven, melted butter does not serve the same roll in leavening pastries as softened and cold butter do. However, it does still play a roll in the texture. For instance, using melted butter in a cookie recipe will make them chewy.
You may also see melted butter in recipes that only require gentle mixing (or the "muffin method" where the dry ingredients to be mixed with the all of the wet), like quick-breads, pancakes, brownies, some cakes, and muffins.
Storing Butter
Although it can stand at room temperature for some time, it is still best to keep butter in the refrigerator if you don't have immediate plans for using it — or even in the freezer. Butter is susceptible to odors, so keep it covered and separate from foods with strong odors, like fish or onions. Exposure to air also quickens the time it will take for the butter to turn rancid. Butter will usually keep for about three months in the refrigerator and about three to four days if left at room temperature.
https://www.thekitchn.com/baking-school-day-6-all-about-butter-and-baking-222484
Lesson one: soft butter
When butter and sugar are creamed together, air is incorporated evenly in the base of a batter or dough. Soft butter creates tenderness and lift.
Use it in: cakes and soft cookies.
For best results: make sure butter is fully at room temperature. If you’re in a hurry, cut butter into tiny pieces and it will quickly soften.
Lesson two: cold butter
Cold butter is ideal for baked goods that should be crisp. Butter that’s straight from the fridge doesn’t get fully incorporated into a batter; instead it gets broken down into small pieces throughout your dough. Since butter is about 18 percent water, steam is released in those pockets during baking, which helps create flaky layers.
Use it in: scones, pie crust, biscuits and crispy cookies.
For best results: grate butter or use a food processor or pastry knife. Handle dough as little as possible, and if it starts to feel warm, pop it in the fridge.
Lesson three: melted butter
Because melted butter has already released much of its water content, it makes the finished treats soft and dense, as well as flavourful. Use it in loaves and brownies.
Use it in: loaves and brownies.
For best results: let melted butter cool to room temperature before incorporating.
http://www.chatelaine.com/recipes/recipe-collections/how-butter-works-why-recipes-call-for-cold-soft-or-melted-butter/