Preheat oven to 350°F. Lightly grease and flour 9x5-inch loaf pan. Mix flour, baking powder, baking soda, salt and cinnamon in medium bowl. Set aside.
Beat butter and sugar in large bowl with electric mixer on medium speed until light and fluffy. Add eggs and banana flavor; mix well. Add bananas and sour cream; beat on low speed until well blended. Gradually beat in flour mixture until well mixed. Stir in walnuts. Pour into prepared pan.
Bake 55 to 60 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pan; cool completely on wire rack.