In a large bowl, mix the flour, light brown and white sugar, and butter in a large bowl. Using your hands or a fork, mix until a crumbly mixture forms and place in fridge as you make the muffin batter. This will be the streusel topping for later.
1/4 cup butter, 3/4 all-purpose flour, 1/4 cup dark brown sugar, 1/4 cup white sugar
In a large mixing bowl, add the pumpkin puree. Then take 2 paper towels and gently press down on the pumpkin, 'blotting' it down and removing some moisture. Do this twice for about 2 minutes.
3/4 cups pumpkin puree
Into that same bowl with the pumpkin, add in the melted butter, sour cream, brown sugar, white sugar, eggs, vanilla, and mashed banana. Using a whisk whisk it until smooth. There may be some chunks from the banana but that is okay.
1/2 cup butter, 1/4 cup sour cream, 1/3 cup banana, 1/2 cup dark brown sugar, 2 eggs, 1 tbsp vanilla, 1/3 cup white sugar
Add in the flour, cinnamon, baking soda, baking powder, and salt. Using a spatula, gently fold until just combined and no dry streaks show. Don't overmix!
2 cups all-purpose flour, 2 tsp cinnamon, 1 1/2 tsp baking powder, 1 tsp baking soda, 1/4 tsp salt
Preheat the oven to 425°F/218℃. Line a 12 muffin pan with 6 muffin liners and bake the muffins 6 at a time. This helps give them a taller rise.
To avoid a mess, scoop the muffin batter using a cookie scoop. The muffin liner should be completely full.
Take the streusel and sprinkle a generous amount on top. Then take some white sugar and sprinkle that on top. This will help give the muffins that nice muffin top bakery style crunch.
Bake the muffins for 5 minutes at 425°F. Then reduce the heat to 375°F and bake for another 11-12 minutes. A toothpick inserted will come out clean.
Slightly open your oven door to allow the hot air to escape. Leave the muffins like this inside the oven for about 5 minutes and then proceed to remove them from your oven completely and place them on your counter to continue cooling.
If you remove the muffins from the oven right away, they tend to drop a lot.
Enjoy!