Preheat the oven to 375℉. Line a muffin liner with 12 liners and set aside. Mash bananas in a large bowl.
1 1/4 cups bananas
In a medium-sized bowl, add in flour, cinnamon, oats, salt, cornstarch, baking powder, and baking soda. Using a spatula, mix until just completely combined.
1 1/2 cups all-purpose flour, 1 cup rolled oats, 1 tsp baking soda, 1 1/2 tsp baking powder, 1 1/2 tsp cinnamon, 1 tsp cornstarch, 1/4 tsp salt
In a large bowl, whisk mashed bananas, oil, eggs, vanilla, sour cream, and dark brown sugar. Whisk until well combined. There may be lumps from the bananas but that is okay.
3/4 cups dark brown sugar, 1 tbsp vanilla, 1/2 cup oil, 1/2 cup sour cream, 1 1/4 cups bananas, 2 eggs
Add the dry into the wet and using a spatula, gently fold until a soft batter forms.
Evenly divide the batter between 12 liners, they should be almost or all the way full. Using a cookie scooper helps big time to minimize the mess. Sprinkle a handful of streusel on top.
Bake the muffins at 375°F for 20-25 minutes. The muffins are ready when a toothpick inserted comes out dry with no wet batter.