Cream together butter or margarine and brown sugar, packed Add 1 egg yolk, sifted flour, and vanilla. Batter will be stiff.
Spread on a greased 13 x 9 inch cookie sheet. Bake at 375 degrees F (190 degrees C) for 12-15 minutes until golden brown.
While still warm, add 1 cup semi-sweet chocolate chips to top of cookie until melted. Spread with knife until top is covered. Add chopped nuts, if desired. Let cool, then cut into squares.
