Preheat oven to 350°F (see note).
In a large bowl, whisk melted butter and ⅔ cup brown sugar until well combined, about 1 full minute.
Add eggs, mashed banana, and vanilla, whisking until combined.
In a separate bowl, combine the flour, baking soda, salt, and cinnamon.
Add dry ingredients to the wet ingredients and use a rubber spatula to lightly stir the batter until almost combined.
Add ½ cup chopped nuts and stir until no streaks of flour remain. Be careful not to over-mix.
Grease or line 15 standard-sized muffin cups and divide the batter evenly among the cups.
Sprinkle the top of each cup with brown sugar and chopped walnuts (topping).
Bake for 14-17 minutes, until a toothpick inserted into the center comes out clean.
Place the pan on a wire rack to cool.
