Preheat the oven to 375℉/190℃. Line a muffin tin with 12 liners and set aside. Mash bananas in a large bowl.
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, 1/4 tsp salt, 1 tsp cornstarch
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, 2 large eggs, 1 1/4 cups bananas, 1/2 cup sour cream
Add the dry and wet ingredients together and using a spatula, gently fold until a soft batter forms.
Evenly divide the batter between the liners, they should be almost or all the way full. Sprinkle some white sugar on top.
white sugar for sprinkling
Bake the muffins at 375°F for 20-25 minutes. The muffins are ready when a toothpick inserted comes out clean.