Preheat your oven to 350°F (175°C) and prepare a muffin pan with paper liners or cooking spray.

In a large mixing bowl, mash the ripe bananas until smooth and stir in the melted butter until combined. Add the beaten egg and vanilla extract, mixing until cohesive.

In another bowl, whisk together the flour, sugar, baking soda, and salt. Gradually fold this mixture into the banana mix until no dry flour is visible.

In a separate bowl, combine oats, brown sugar, cinnamon, and melted butter until crumbly. Set this aside.

Fill each muffin cup with batter about two-thirds full and generously sprinkle the crumble topping on each.

Bake for 18-20 minutes, checking for doneness with a toothpick.

Remove from the oven and cool in the pan for 5 minutes before transferring to a wire rack to cool completely.