Cream butter and sugar. Add unbeaten egg yolks.
Sift flour and baking powder together and add, a little at a time, to the first mixture, stirring after each addition.
Let dough rest overnight in the refrigerator. In the morning, roll it about ½ inch thick on a lightly floured board, or flatten it out with your hands.
Cut into squares and place on greased and floured cookie sheets. Bake in a preheated 350° oven 12 to 15 minutes.