Preheat your oven to 425F (220C). Line a muffin tin with paper liners.
In a large bowl, mash the bananas until smooth.
Add the avocado oil and melted butter to the mashed bananas. Stir well.
Stir in both the sugar and light brown sugar until well-combined.
Add eggs, buttermilk, and vanilla extract to the banana mixture and stir well.
In another bowl, whisk together flour, baking powder, baking soda, and salt.
Gradually fold the dry ingredients into the wet mixture until just combined.
Portion the batter into the prepared muffin tin, filling each liner about ¾ full.
If using, combine the streusel topping ingredients and sprinkle over each muffin.
Bake the muffins for 8 minutes at 425F, then reduce the temperature to 350F (175C) and continue baking for another 7-8 minutes.
Use a toothpick to test for doneness, then cool in the tin for a few minutes before transferring to a wire rack.