Cream together butter and sugar. Add egg yolks and vanilla. Sift dry ingredients together and gradually stir in. Press into a greased 9x13 baking pan. For topping, beat egg whites and beat in brown sugar. Spread over top of dough. Cover with walnuts. Bake approx. 45 minutes at 300°. This recipe came from a former secretary of Jack Thorpe's at Thorpe Sullivan -- Mary something. It's a delicious, but rich, coffee cake.