Preheat oven to 425F (220C) and line a muffin tin with paper liners.

Place peeled bananas in a large bowl and use a fork or a potato masher to mash them — bananas should be well-mashed.

Add oil and melted butter and stir well.

Stir in sugars until well-combined.

Add eggs, buttermilk, and vanilla extract and stir well. Set aside.

In a separate bowl, whisk together the dry ingredients — flour, baking powder, baking soda, and salt.

Add dry ingredients to the wet batter and gently fold in until ingredients are just combined — don't over-mix the batter!

Portion batter into prepared muffin tin, filling each liner ¾ of the way full. Set aside and prepare the Streusel topping.

Combine flour, brown sugar, and salt in a medium-sized bowl and use a fork to stir the ingredients together.

Using a pastry cutter, cut in the cold butter until the butter is well-worked in and the mixture resembles coarse crumbs.

Sprinkle streusel topping evenly over each muffin.

Bake muffins on 425F (218C) for 8 minutes and then reduce the oven temperature to 350 (175C) (do not open the oven door or remove the muffins, just leave them in and reduce the temperature) and bake another 12-13 minutes on 350F (177C). A toothpick inserted in the center of the largest muffin should come out clean or with few moist crumbs (no wet batter) to test for doneness.

Allow muffins to cool before enjoying.