Preheat oven to 350℉. Prepare your muffin tray by lining each cup with a muffin cup liner.
Mash the three bananas well with a fork or masher, and set aside.
In a mixing bowl, mix the flour, baking powder, baking soda, and cinnamon.
In another bowl, mix the melted butter and ⅔ Cup brown sugar, leaving the remaining brown sugar aside to top the muffins.
Mix the eggs, banana, and vanilla extract to the butter and sugar mixture.
Add in the flour mixture and mix until almost, but not quite, fully combined.
Add ½ cup of chopped nuts and fold in, which will finish mixing the batter. Try not to over-mix or you'll make your muffins tough.
Spoon your batter into your muffin cups, filling each about ¾ of the way full.
Top each muffin with brown sugar and nuts.
Bake for 17-20 minutes, then remove and let cool for 5 or so minutes before enjoying.