The perfect pie begins with the perfect crust-one that is tender and flaky. Cutting the butter into the flour mixture is a key step. The butter must be cold and hard; if it warms up and softens, the flour will absorb it, become sticky and yield a dense, tough crust. We recommend mixing the ingredients in a food processor. It allows you to work quickly so the butter doesn't have a chance to soften. And by running the machine in short pulses, the processor won't heat up and melt the butter.