Cream butter and sugar together. Add yolks one at a time, beating well after each addition. Add sifted dry ingredients, alternating with milk and vanilla. Add in thirds, beating until smooth after each addition. Grate chocolate coarsly, reserving one third cup for the topping. Beat whites until stiff and fold into batter, add remaining chocolate to batter and mix. Pour in a greased and floured 9x13 pan. Sprinkle the top with the one third cup of grated chocolate. Bake in a preheated 350oF oven for 40 minutes.