Combine all ingredients in a bowl and use a fork to mix until all ingredients are evenly coated with butter. Let the mixture remain in small clumps and set aside.
To make the banana muffins:
Preheat oven to 400°F. Prepare two muffin pans with lotus-style paper liners (or regular paper liners) by placing one liner in every other cavity. If you’re using lotus-style liners, this recipe makes about 10 large muffins. If you’re using regular paper liners, it will probably make a few more.
Combine flour, baking powder, salt, cinnamon, and both sugars in a large bowl; whisk to combine. Add egg, oil, milk, vanilla, and mashed banana. Stir until dry ingredients are incorporated, but a few lumps are okay.
Spoon muffin batter into the prepared paper liners about ¾ full. Add a heaping spoonful of the crumble mixture on top, making sure to cover all of the batter.
Bake muffins for 5 minutes at 400°F, then lower the oven temperature to 350°F and bake for 15 to 18 minutes, or until a toothpick inserted into the center of the muffins comes out clean (a few crumbs are okay).
Remove from oven and cool in pans for 10 to 15 minutes, then transfer to a wire rack to cool completely. Store muffins in an air-tight container at room temperature.