Preheat oven to 425°F/218°C. Line a 12-well muffin tin with 6 liners with one space between each and set aside.
Whisk the flour, baking powder, cinnamon, and salt in a medium mixing bowl and set aside.
1 ¼ cup all purpose flour, 1 ½ teaspoon baking powder, ½ teaspoon ground cinnamon, ½ teaspoon sea salt
Add the melted butter, oil, granulated sugar, brown sugar, egg, and vanilla to a separate large mixing bowl and whisk until well combined.
2 tablespoon unsalted butter, 2 tablespoon vegetable oil, ½ cup granulated sugar, 2 tablespoon brown sugar, 1 large egg, ½ teaspoon vanilla extract
Whisk in the mashed banana and buttermilk until everything is smooth with few lumps.
1 mashed banana, ¼ cup buttermilk
Fold the dry ingredients into the wet ingredients using a rubber spatula until no dry streaks of flour remain. It's ok if there are some lumps.
Scoop the batter into each muffin liner. Fill each liner ¾ full and sprinkle coarse sugar on top if desired.
Bake at 425°F/218°C for 5 minutes, reduce the heat to 400°F/204°C, and bake for 10-15 more minutes. The muffins are done when the tops are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Remove from the oven and let cool on the counter for 10 minutes. Take the banana muffins out of the pan and transfer them to a wire cooling rack to cool for 30 more minutes and enjoy.