Preheat oven to 350 degrees. Make the cake first, combining dry ingredients in a large bowl. Add eggs and salad oil, stirring by hand until dry ingredients are moistened. Stir in vanilla, pineapple, bananas, and one cup of crushed walnuts. Stir only with a spoon. Do not use an electric mixer. Spoon batter into three well-greased and floured 9" round cake pans. Bake until cake is done. Cool in pans for 10 minutes. Remove cakes from pans and cool on wire rack. Meanwhile, make the frosting by beating butter and cream cheese with an electric mixer until light and fluffy. Add confectioner's sugar gradually, beating until at spreading consistency. Beat in vanilla. Frost each layer, assembling. Finish frosting cake and sprinkle remaining chopped walnuts on top.