Cream butter and brown sugar until light.
Add egg yolk and vanilla and beat until well blended.
Sift dry ingredients together and stir into sugar mixture (dough will be stiff).
Spread in 10x15" pan and bake at 350 degrees for 15-20 minutes.
Remove from oven and sprinkle with chocolate pieces until the chocolate melts.
Spread evenly over the surface.
Sprinkle with nuts.