Sift flour once, measure, add baking powder, salt and sugar, and sift together three times. Add 4 tablespoons butter. Combine egg, milk, and vanilla and add to flour mixture, stirring until all flour is dampened. To melted chocolate, add boiling water, soda, and 1 tablespoon butter; cool slightly. Add to cake batter and beat vigorously 1 minute. Bake in greased tube pan in moderate oven (350 degrees F.) 50-60 minutes or until done. Serve warm or cold with Chocolate Walnut Cream Filling.