Black Walnut Cake

1/2 cup butter
2 cups firmly packed brown sugar
3 egg yolks, beaten
3 teaspoons baking powder
2 cups all-purpose flour
1/2 teaspoon salt
2/3 cup milk
1 teaspoon vanilla extract
1 cup black walnuts, finely chopped
3 egg whites, stiffly beaten

Combine butter and sugar and beat until smooth. Add beaten egg yolks and mix well. Combine dry ingredients and add to creamed mixture, alternately with milk. Add vanilla extract and walnuts and mix well. Fold in stiffly-beaten egg whites. Bake in greased 9- or 10-inch tube pan at 350 degrees F or until cake is done.