1. Cream butter, then cream in sugar, egg yolk and vanilla.
2. Gradually add flour, mixing well. 3. Spread in a greased 9 x 12 pan and smooth with the back of a spoon.
4. Bake at 325° for 25-30 minutes.
5. Grate chocolate bar, then sprinkle the chocolate over the pastry. Even off with a spoon or knife.
6. Sprinkle with nuts.
7. Cut while warm.