Preheat oven to 350 degrees F. Cream together the butter and sugar. Stir in the egg and vanilla. Spread this dough into a large jelly roll pan. Bake for about 15 minutes until the top is slightly brown.
Remove from the oven and immediately arrange the Hershey bars on top. As the chocolate melts, spread it to cover the bottom layer completely. Sprinkle with nut to cover, press these gently into the chocolate with a spatula.