Beat egg yolks in a small mixing bowl until thick and lemon colored. Set aside. Beat egg whites (at room temperature) in a medium mixing bowl until foamy; set aside.
Combine flour, sugar, and baking powder in a large mixing bowl; mix until well blended. Stir in butter, reserved egg yolks, and vanilla. Fold in egg whites, blending well. Cover and refrigerate overnight.
Shape dough into 1-inch balls.
Bake, a few at a time, in a preheated, lightly oiled waffle iron until golden brown. Cool on wire racks, and store in airtight containers.
Note: Dough may be shaped into balls, frozen, and thawed to bake when desired.
