Preheat oven to 350 degrees. Grease a loaf pan (9x5). In a large bowl, cream together the butter and sugar until light and fluffy. Stir in the eggs one at a time, beating well. Stir in vanilla and banana. In a separate bowl, sift together flour, baking powder, and salt. Blend the banana mixture into the flour mixture; stir just to combine. Fold in walnuts. Pour batter into greased pan. Bake for 50-60 minutes.