• 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.
  1 ½ cups well-mashed ripe bananas
• Add oil and melted butter and stir well.  
  ¼ cup avocado oil or vegetable oil or canola oil, ¼ cup unsalted butter
• Stir in sugars until well-combined.
  ½ cup sugar, ½ cup light brown sugar
• Add eggs, buttermilk, and vanilla extract and stir well. Set aside.
  2 large eggs, 1 Tablespoon vanilla extract, ¼ cup buttermilk
• In a separate bowl, whisk together your dry ingredients -- flour, baking powder, baking soda, and salt.
  2 cups all-purpose flour, 1 ½ teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt
• Add dry ingredients to your 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 your Streusel topping.

Streusel Topping (optional)

• Combine flour, brown sugar, and salt in a medium-sized bowl and use a fork to stir the ingredients together.
  ½ cup all-purpose flour, ½ cup brown sugar, ¼ teaspoon salt
• Using a pastry cutter, cut in your cold butter until the butter is well-worked in and your mixture resembles coarse crumbs.
  ¼ cup unsalted butter, cold
• 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 7-8 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.