1. Cream together butter & brown sugar.
2. Add flour, egg yolk and vanilla. Mix well.
3. Press dough onto cookie sheet about 1/8 inch thick.
4. Bake at 300 to 325 degrees. Watch carefully. when just done, remove from oven. Do not let it brown.
5. Melt Hersey's bars in double boiler and spread over top of cookie mixture; putting walnuts over top. Cut before cold.