In a small bowl, cream butter and sugar. Beat in the egg, banana and vanilla. Combine the flour, baking powder, salt, baking soda and cinnamon; add to creamed mixture just until moistened. Fold in walnuts.
Coat muffin cups with cooking spray or use paper liners; fill two-thirds full with batter. Bake at 350° for 23-25 minutes or until a toothpick comes out clean. Cool for 5 minutes before removing from pan to a wire rack. Serve warm.