Cream the butter and sugar smoothly together; add the egg yolks one at a time, beating after each addition. Add the vanilla, then gradually the flour, which has been sifted with the baking powder. Beat continually. Whip 3 egg whites stiff with the salt and fold in. Pour into a buttered loaf or funnel pan. Bake in a moderate oven (350°F.) 50 minutes to 1 hour. Test for doneness.
Use the other egg white to make a simple icing for the top; or use it in the cake recipe.