Preheat the oven to 425℉. Line 2 muffin tins with 14 liners total and set aside. Mash bananas in a large bowl and set aside.
1 1/4 cups bananas
In a medium-sized bowl, add in the dry ingredients. Mix with a spatula until combined and set aside.
1 1/4 cups all-purpose flour, 1/4 cup unsweetened cocoa powder, 1 1/2 tsps baking powder, 1 tsp baking soda, 1 1/4 cups rolled oats, 2 tsps cornstarch, 1/4 tsp salt
In a large bowl, whisk mashed bananas, oil, eggs, vanilla, sour cream, and brown sugar. There may be lumps from the mashed bananas.
3/4 cups dark brown sugar, 1 tsp vanilla, 1/2 cup oil, 2 large eggs, 2/3 cups full-fat sour cream, 1 1/4 cups bananas
Add the dry and wet together. With a spatula, fold until a soft muffin batter forms. Do not over-mix!
Add the chocolate chips in and mix in a few times. Don't over-mix!
3/4 cups semi-sweet chocolate chips
Evenly divide the batter between the liners, they should be almost or all the way full. Sprinkle some extra chocolate chips on top.
Bake the muffins at 425℉ for 5 minutes, then reduce the temperature to 375°F and continue baking for 15-20 minutes. The muffins are ready when a toothpick inserted comes out dry with no wet batter.