Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Reserve 1 tablespoon of the cake flour. Sift together remaining flour and baking powder. Set aside.
Toss the chocolate and walnuts with reserved 1 tablespoon of flour. Set aside. In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Fold in the chocolate and walnuts.
Pour batter into prepared pan. Bake in the preheated oven for 35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool before frosting.