Preheat the oven to 375℉. Spray a metal, nonstick muffin pan with nonstick spray. The batter "might" make more than 12 muffins.
PREP THE MUFFIN BATTER:
In a medium bowl, whisk together the flour, baking powder, baking soda and salt; set aside.
In a large bowl, whisk together the sugars with the oil. Then whisk in the buttermilk, eggs and vanilla extract. Then stir in the mashed banana. Stir in the dry ingredients, just until all ingredients are incorporated.
Add the batter to the muffin pan. Fill each muffin cup to about ¼-inch from the top. They should each rise nicely.
ADD THE CRUMBLE:
In a medium bowl, use a pastry blender to combine the crumble ingredients. Sprinkle the crumble on top of each of the muffin batter.
Bake for about 18 minutes. Check for doneness. Bake longer, if needed.