Heat oven to 350 degrees. Grease or coat a 9x5 inch loaf pan with cooking spray.
In a blender, blend the bananas until a smooth consistency. Set aside.
With an electric mixer, cream butter and sugar together until light and fluffy.
Add the eggs, one at a time, beating well.
Combine the all-purpose flour, baking soda, salt and cinnamon. Add to butter mixture, about 1/4 cup at a time, and stir to blend.
Stir in the whole-wheat flour, then add the bananas and vanilla until just combined.
Pour the batter into the prepared pan and shake slightly back and forth to even it out.
Bake about 50-60 minutes, or until tester comes out clean. Let stand in pan about 10 minutes before turning out onto rack to cool completely.
Note: For medium-sized loaves, reduce baking time to about 30-35 minutes. For mini-loaves, reduce baking time to about 25 minutes.