Cream butter and shortening. Gradually add sugar, beating until light and fluffy and sugar is dissolved. Add egg yolks and beat well.
Combine soda and buttermilk; stir until soda is dissolved. Add flour to creamed mixture alternately with buttermilk mixture, beginning and ending with flour. Add vanilla, walnuts and coconut; stir thoroughly.
Beat egg whites (at room temperature) with cream of tartar until stiff peaks form, then fold into batter. Pour batter into 3 greased and floured 9-inch cake pans. Bake at 350 degrees for 30 minutes, or until cake tests done.
Cool in pans 10 minutes, then remove from pans and finish cooling. Ice with Black Walnut Butter Cream Frosting.