Preheat the oven to 350°F. Line a 12-count muffin pan with cupcake liners.
In a medium bowl, whisk the flour, baking powder, baking soda, salt and cinnamon. Set aside.
In a large bowl, mash the bananas. Whisk in the eggs, brown sugar, melted butter and vanilla. Pour the dry ingredients on top of the wet ingredients and continue to whisk until combined. Fold in the chopped walnuts.
Pour the batter into the prepared muffin pan and bake for 18-20 minutes or until a toothpick comes out clean.
Notes
Store at room temperature in an airtight container for up to 4 days, in the fridge for 5-6 days, or individually wrapped in the freezer for up to 3 months.