- 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.