In a mixing bowl, cream together butter and sugar. Add eggs, one at a time, until well blended. Stir in vanilla and bananas. In a separate bowl, combine flour, baking powder, baking soda and salt. Add to banana mixture alternately with sour cream. Stir in nuts. Pour into a greased and floured 10-inch Bundt pan.
Bake in a preheated 350 F oven for 50 minutes or until a toothpick comes out clean. Cool, and then dust with powdered sugar before serving.