Preheat oven to 375 degrees.
Combine flour, baking soda, salt, and cinnamon in a medium bowl. Set aside.
Mash 3 bananas with a fork in a large bowl and when they are mashed, add in the sugars, butter, eggs and vanilla.
Mix with an electric mixer on low speed.
Fold in ½ cup of nuts with a spatula. Save a few chopped walnuts to sprinkle on top of the muffin batter at the end.
Add dry ingredients from medium bowl to the wet ingredients and stir in with a spatula.
Pour batter into the cups of a greased muffin tin. You can also use cupcake liners if you wish. Divide batter among the 12 cups.
Sprinkle remaining walnuts on top of the batter.
Bake at 375 degrees for 20-22 minutes or until a toothpick inserted into the center comes out clean.