Preheat oven to 350 degrees. Spray bottom only of 9x5x3 inch loaf pan with nonstick cooking spray.
Beat butter in large bowl with an electric mixer set at medium speed until light and fluffy. Add granulated sugar and brown sugar. Beat well. Add egg, egg whites, and vanilla. Beat until well blended. Add mashed banana and beat on high speed 30 seconds.
Combine flour, baking soda, salt, and baking powder in medium bowl. Add flour mixture to butter mixture alternately with cream, ending with flour mixture. Add walnuts to batter. Mix well.
Pour batter evenly into prepared loaf pan. Bake until browned and toothpick inserted near center comes out clean, about 1 hour 15 minutes.
Cool bread in pan on wire rack 10 minutes. Remove bread from pan. Cool completely on wire rack. Can be eaten right away, but I think it's even better if the loaf is wrapped in plastic wrap overnight to let the flavors develop.