Butter Cookie Recipe

3/4 cup butter or margarine, softened
1/4 cup granulated sugar
1/4 cup packed light brown sugar
1 egg yolk
1 3/4 cup all purpose flour
3/4 teaspoon baking powder
1/8 teaspoon salt

Combine butter, granulated sugar, brown sugar and egg yolk in medium bowl. Add flour, baking powder and salt; mix well. Cover; refrigerate until firm, about 4 hours, or overnight.