1 c. butter
1 egg yolk (I use whole egg)
2 c. flour
1 c. brown sugar (packed)
1 tsp. vanilla

Cream together all of above ingredients until blended. Spread mix on 10 1/2 x 15 1/2 pan, ungreased. Bake 25 minutes at 350 degrees.

Meanwhile, melt giant sweet chocolate bar (9 3/4 ounces) in double boiler or in microwave. While cookies are hot from oven, spread on melted chocolate. Sprinkle with finely chopped walnuts or pecans (about 1/2 cup). When cool, cut in squares.