In a small bowl, beat egg whites until soft peaks form; set aside. In a large bowl, cream shortening and sugar. Beat in egg yolks; mix well. Add bananas. Combine flour and baking soda; add to creamed mixture alternately with buttermilk, beating well after each addition. Add vanilla. Fold in egg whites. Fold in nuts if desired. Pour into a greased 13-inch by 9-inch baking dish. Bake at 350° for 45-50 minutes. Cool on a wire rack. If desired, garnish with whipped cream and a few banana slices.