To a medium bowl, add all-purpose flour, granulated sugar, brown sugar, ground cinnamon, baking powder, baking soda and salt. Whisk until combined.
To a large mixing bowl, combine mashed bananas, oil, butter, eggs, buttermilk, and vanilla extract. Stir until combined well.
Using a spatula, gradually mix the dry ingredients into the wet ingredients just until combined. Rest at room temperature for 15 minutes. While it rests, preheat the oven to 425ºF and make the streusel.
In a small bowl, whisk together all-purpose flour, brown sugar, and ground cinnamon. Cut the butter into the mixture with a pastry cutter or fork until it's resembles small crumbs.
Grease a mini muffin pan with butter or nonstick spray. Using a cookie scoop or spoon, fill each muffin to just below the edge of the muffin well.
Sprinkle the crumb topping over the muffins.
Bake for 5 minutes at 425ºF. Leaving the door closed, turn the temperature down to 350ºF and bake for 5-7 minutes more or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool 15 minutes before transferring to a wire rack.