Cream butter and sugar in an electric mixer. Add eggs and vanilla extract; mix until blended. Add bananas. Mix in flour mixed with baking powder and cinnamon until just blended. Stir in walnuts.
Spread mixture in a greased 9 x 5-inch loaf pan. Bake 55 to 60 min. at 350°F or until toothpick inserted in center is clean when removed. Let sit for 10 min., loosen edges from pan with a knife, remove from pan and cool on a wire rack.