Smash or mix overripe bananas in a bowl with a fork or hand mixer.
Add all ingredients above through the baking powder to the same bowl.
Slowly mix in the flour until you have a thick pancake batter consistency. You might need slightly more or less flour based on the size and moisture content of the bananas.
Melt the butter in the microwave or on the stovetop and mix in cinnamon to taste. I am crazy about cinnamon and think you never have too much, so add to your heart's content!
Add walnuts and 2 TBSP of sugar to the food processor to combine.
To assemble:Pour half the batter into a greased or lined 9-inch loaf pan. Then drizzle the butter-cinnamon mixture on top. Use a butter knife to swirl the butter mixture into the loaf. Gently fold the butter into the loaf with a few passes of the knife. You want to create a swirl effect, not fully mix. Pour the rest of the batter into the loaf pan and repeat the step above with the butter mixture. Top with the walnut and sugar mixture. Tent with tin foil and bake for about 55-65 minutes. Check at 50 minutes with the toothpick test!Leave the tin foil off for a few minutes at the end to toast the walnuts on top, but watch it carefully so as not to burn them! Let it cool for 10-15 minutes before serving, and enjoy!