In a large bowl with an electric mixer, beat butter and brown sugar until fluffy

Mix in egg yolk, flour and vanilla

Spread dough into a 15x10x1" jelly-roll pan and pat to about 1/4" thickness

Bake in a preheated 325 degree oven about 30 minutes

Remove from oven

While still hot, sprinkle chocolate morsels on top

With table knife, spread melting chocolate to cover surface

Sprinkle walnuts over chocolate

Cut into bars while still warm

Note: These cookies freeze well. Layer bars in a ridged container with waxed paper between layers.