Preheat the oven to 425 degrees and line a muffin or cupcake tray with 11-12 liners (see note).
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
2 cups flour, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt, 1 ½ teaspoons cinnamon
In a separate medium bowl whisk together the melted butter, eggs, vanilla extract, sugars, pumpkin puree, and mashed banana. Slowly add wet ingredients to dry, using a spatula to gently combine, ensuring not to overmix.
½ cup butter, 2 eggs, 1 teaspoon vanilla extract, ½ cup granulated sugar, ¼ cup light brown sugar, ¾ cup pumpkin puree, ½ cup banana
Divide the batter between the 11-12 liners using a cookie scoop, filling each to the top. Top each muffin with some of the crumble.
Bake the muffins for 10 minutes at 425 degrees, then without removing the muffins from the oven, reduce the heat to 375 degrees and continue baking for 6-7 minutes until the centers rise and a toothpick inserted in the center comes out clean.
Cool the muffins in the pan for 10 minutes, then remove them to a wire rack to continue cooling completely. Enjoy!