Preheat oven to 325°. Cream butter and sugars until fluffy. In small bowl, combine flour and cocoa. Gradually add dry ingredients to creamed mixture; stir just until blended (do not overmix). Press into bottom of lightly greased 9 x 13-inch baking pan. Prick dough with a fork. Bake 23 to 25 minutes, or until edges are lightly browned. Cool in pan on a wire rack 10 minutes. Cut warm shortbread into 1 x 2-inch bars. Cool completely in pan. Makes about 4 dozen bars.