An hour before you cook the cookies, put the butter in the freezer. It will be easier to grate, and then to rub it with flour into the crumbs. You ask why we need these crumbs? In fact, you and I are "rubbing" the fat into the flour, while leaving the butter in rather large pieces. When we roll out the dough, the butter and flour will form layers. And then in the oven, the liquid from the butter will lift the dough and create appetizing layers.