Rubbing butter or another type of fat into flour or dry ingredients is a common first step in many recipes such as pie dough, scones, and cookies. Under or over mixing the fat can lead to less than perfect results. For all of these purposes you want to use cold fat. If flakiness is something that is of the utmost importance, you can even freeze and pre-cut the fat beforehand.
The next step is to cut the cold fat into small chunks — rough 1-2 cm cubes are best. If you want to be very dedicated, you can chill everything from the flour to the bowl. Cold hands are best, but working fast will compensate for this if you don’t’ feel like rubbing ice cubes on your wrists until they are painfully cold. You can cut the fat into the dry ingredients using forks, hands, a pastry cutter (a special tool for just this purpose), or even the paddle on an electric mixer. What you’re trying to avoid is gluten development which is what makes doughs tough and chewy.
Two things need to be present for gluten development, flour and moisture. While butter does contain a certain amount of moisture, there is not a sufficient amount to develop gluten. When mixing, I like using either my hands or an electric mixer as forks are too messy and hard to control, and a pastry cutter is another gadget that only does one thing (Alton Brown’s much hated unitaskers!). When you mix with your hands, put the cold cubed butter into the bowl, and stir it around gently just to get all the chunks coated in flour. Now you can start to rub the chunks between your thumb and first 2 fingers. Don’t rub too much, just enough to break the chunk of fat up. Any larger pieces can be come back to.
The mixer method is basically the same with a slight difference. You will need to, after a couple of minutes, turn the mixer off and scrape the bottom to make sure all the flour gets a chance to be coated in fat.
Eventually after enough rubbing or mixing, the mix will change colour becoming more yellow and the texture will become more and more fine. For items like flaky pie pastry and scones, stop when the fat is still in visible chunks the size of your little fingernail. For less flaky doughs, continue cutting in the dough until it is the texture of soil or sand. During baking, the fat will melt trapping the moisture released from the butter or shortening which will leaven the item.
