Cream butter in a large mixing bowl; gradually add sugar, beating until light and fluffy. Add egg yolk; beat well. Stir in almond extract. Gradually add flour and ground almonds, stirring until well blended.
Press dough into a lightly greased 13- x 9- x 2-inch baking pan. Bake at 325° for 40 minutes or until lightly browned. Cut into 2-inch squares. Cool in pan 10 minutes; remove to wire racks to cool completely.