Cream together butter, eggs, and sugar. Add vanilla and buttermilk.
Mix dry ingredients together and add to the creamed mixture.
Add bananas and 1/2 cup chopped walnuts. Mix well.
Pour into 2 (or 3 small) well-greased cake pans.
Bake at 350 for 35-40 minutes.
Cool for 15 minutes and remove from pans. Frost the cake and serve!