Preheat the oven to 350°F. Spray muffin pans with cooking spray and set aside. (See notes below).
In a medium bowl, mix together all of the oatmeal topping ingredients and set aside.
In another medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large bowl, whisk together the mashed banana and the eggs. Add the buttermilk, oil, sugar, and vanilla and mix well.
Stir the dry ingredients into the wet ingredients until just combined. Do not over mix.
Portion out the batter into your muffin pans. I fill my wells about 3/4 full. Top each muffin with oatmeal topping.
Bake for 15-20 minutes. When a toothpick is inserted into the center of the muffins, it should come out moist but with no raw batter on it.
Let the muffins cool in the pan for about 5 minutes. Remove the muffins from the pans and place on a cooling rack to cool completely. If your pans are prone to sticking, make sure to loosen up any crumb topping on the muffin pan before it cools completely.