Often a recipe will call for you to "cut in" butter or shortening—usually when making biscuits, scones, or some other pastry that needs to be flaky. "Cutting in" means incorporating the butter into the flour in such a way that little lumps of the raw butter remain whole within the flour mixture. When the dough is baked, these little lumps create separation in the structure of the finished product, which is what gives it that flaky consistency.
The easiest way to cut in butter is with the simple tool called a pastry blender.
Some bakers chill everything—the butter, the flour, even the bowl and other tools. Why? Flour contains proteins called glutens that stiffen up as a dough is mixed or kneaded. Cool temperatures slow down this stiffening, giving the baker more control over the process.
When butter is warm, it softens and blends in with the flour, so you get fewer of the little lumps and thus a less flaky texture, which is not what you want. Therefore making sure the butter is cold is a key step to perfect pastry.
It's important to measure your flour accurately because it is the ratio of butter to flour, and the way the lumps of butter blend in with the flour, that creates the flaky texture you want. Sifting the flour helps ensure a uniform amount when you're measuring by volume. Unlike with liquids, the amount of flour in a cup depends on how tightly it's packed; a loosely packed cup has more flour in it than a tightly packed cup. Sifting helps eliminate that discrepancy to some extent, and that's important because if there's too much flour, the butter-to-flour ratio will be off, and your pastry won't be as flaky.
Ultimately, though, because baking is so precise in its ratios, professional bakers specify ingredients in weights rather than volumes. That way no matter whether the flour is sifted, tightly packed, or somewhere in between, a pound is always a pound.
The term "pastry blender" can be misleading and sounds like it should be some sort of electric kitchen appliance or at least something with moving parts. Instead, it is a very basic and simple device, and quite affordable averaging less than $5. Some people like to use a fork, or a pair of knives, or even their fingers, but a pastry blender makes it much easier. The problem with doing it by hand is that your fingers will warm up the butter too much. You may want to chill the pastry blender beforehand.
To use the pastry blender, grip the handle and press the blades down into the butter, essentially cutting the butter into pieces. Twist the blender a half-turn and then lift up and repeat several times in quick motions until the mixture is the right consistency.
Some recipes will specify how big the lumps of butter should be. One might call for "pea-sized" lumps while another may say the flour mixture should resemble crumbs. Still, other recipe directions will suggest a consistency resembling cornmeal. No matter what, just remember, the flakier you want your pastry, the bigger the lumps of butter need to be.
When finished, the lumps of butter should still be visible in the flour mixture. Lumps this size would qualify as "pea-sized."
At this stage, you can refrigerate the dry ingredients with the butter cut in and hold it for baking later. Once you add any wet ingredients like water, eggs, or milk, you've got to finish the recipe and bake it away then as the moisture will activate the leavening agents (baking powder or baking soda).