Preheat oven to 175°C (347°F). Line a 12-cup muffin pan with paper liners.
In a bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.
In a separate bowl, mash the bananas well. Stir in the egg, sugar, melted butter, and vanilla until combined.
Fold the dry ingredients into the banana mixture, mixing just until no streaks of flour remain. Avoid overmixing.
Divide the batter evenly among the 12 muffin cups, filling each about ¾ full.
For the topping, combine the brown sugar, flour, and oats in a small bowl. Add the softened butter and work it in with a fork or your fingertips until the mixture looks like wet sand. Stir in the walnuts.
Sprinkle a generous amount of crumb topping over each muffin.
Bake for 18–20 minutes, until golden on top and a toothpick inserted into the center comes out clean.
Cool in the pan for a few minutes before transferring to a wire rack. Let cool completely before storing.