Preheat oven to 350°F (180°C). Line a 12-cup muffin pan with paper liners.
In a measuring jug, combine flour, baking soda, baking powder, salt, and cinnamon. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, add bananas and mash at medium speed. Then add melted butter and sugar. Mix until combined.
Add eggs and vanilla extract, beat at medium speed until incorporated.
Add the dry ingredients to the wet ingredients and mix until just combined and no streaks of flour remain. Fold in walnuts using a spatula.
Divide batter between prepared muffin cups, filling them to the rim.
For the topping: Melt the butter in a skillet, add walnuts and maple syrup. Let simmer at medium-low for 2 minutes until thickened. Add about 1 tsp of topping on top of each muffin.
Bake for 20 - 25 minutes, until a toothpick, inserted into the center, comes out with only a few moist crumbs attached.