For the crumb topping, stir the flour, sugar, and cinnamon together in a medium bowl.
½ cup all-purpose flour, ¼ cup granulated sugar, ½ teaspoon ground cinnamon
Cut in the butter with a pastry cutter (or fork) it forms small pebble-sized crumbs.
¼ cup butter
Chill the crumb topping while you make the muffin batter.
To make the muffins, preheat oven to 375 F. Grease a standard 12-cup muffin tin with nonstick baking spray or line with muffin liners. I like to use a baking spray with flour or parchment paper liners.
In a large bowl, mash the bananas until well-mashed.
1 ½ cups mashed overripe banana
Stir in the melted butter and sugars until well-combined.
½ cup granulated sugar, ¼ cup brown sugar , ⅓ cup butter
Add the egg and vanilla, and mix in until combined.
1 large egg, 1 teaspoon vanilla extract
Stir in the flour, baking soda, baking powder, and salt just until combined.
1 ½ cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, ½ teaspoon salt
Divide the muffin batter among the prepared muffin tin cups.
Sprinkle the crumb topping over the batter.
Bake for 15-20 minutes, or until a wooden toothpick inserted in the center comes out clean.
Let the muffins cook in the pan for 5-10 minutes and then remove to a wire rack to finish cooling.
Notes
Butter: I use salted butter in this recipe. If you're using unsalted butter, add a pinch of salt to the topping, and increase the salt in the muffins by 1/4 teaspoon.
Bananas: I like to use bananas that are equal parts speckled and yellow in this recipe.
Brown sugar: I use light brown sugar, but dark brown sugar also works well in this recipe.