Preheat oven to 350 degrees. Line a 9x13-inch pan with foil.
In a medium bowl, cream together butter or margarine, brown sugar, egg yolk and vanilla until light and fluffy. Add flour, beating until well blended. Spread mixture evenly in foiled- lined pan.
Bake 15-18 minutes or until golden. Sprinkle chocolate pieces evenly over cookie surface. Bake 2-3 minutes more or until chocolate melts. Remove from oven and let stand 1 minute. With a knife, spread melted chocolate evenly over top. Sprinkle with nuts. While warm, cut into squares. Cool in pan and store in the refrigerator, if desired.