Cutting in is a baking technique that is used most often when making pastry dough and pie crust. When you take a cold, solid fat (usually butter) and combine it with a dry ingredient (usually flour) the fat particles become coated in the flour thus preventing gluten forming proteins from joining together which create a tough dough. Cutting in helps to create tiny pockets of butter which melt during the baking process and create a light, flaky crust.
To cut in butter, simply cut up very cold butter into small, even pieces and sprinkle them over the flour. Using a pastry blender (a tool with 5-6 curved parallel blades) OR two knives combine the butter and flour together. With a pastry blender, rock gently back and forth until the mixture becomes crumbly. Scrape the blades as needed while combining. If using knives, hold one in each hand, blades touching, and cut though the mixture creating an “X”. Continue this motion until the mixture becomes crumbly.
When completely combined the mixture should resemble a dry, coarse meal with tiny pea sized pieces of butter throughout. If during the mixing process the butter becomes warm and greasy, place the bowl in the fridge for about 10-15 minutes to get it cold again and then continue mixing until combined.