Preheat oven to 300°. Cream butter and sugar until light and fluffy, 5-7 minutes. Combine flours; gradually beat into creamed mixture. Press dough into an ungreased 13x9-in. baking pan. Prick with a fork.
Bake until light brown, 45-50 minutes. Cut into 48 bars or triangles while warm. Cool completely on a wire rack.
