Prep the pan and oven. Heat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease the cups.
Whisk the dry ingredients. In a medium bowl, whisk flour, baking powder, baking soda, salt, and cinnamon until evenly combined.
Mash the bananas. In a large bowl, mash the bananas until mostly smooth with a few small lumps left for texture.
Mix the wet ingredients. Add granulated sugar, brown sugar, oil, melted butter, eggs, and vanilla to the bananas.
Whisk until the mixture looks smooth and glossy.
Combine wet and dry. Add the dry ingredients to the banana mixture. Stir gently with a spatula until just combined. Do not overmix; a few streaks of flour are fine.
Fold in extras. If using nuts or chocolate chips, fold them in with a few light strokes.
Portion the batter. Divide the batter evenly among the muffin cups, filling each about 3/4 full. Sprinkle with coarse sugar if you like a crunchy top.
Bake. Bake for 16–20 minutes, until the tops are set and golden and a toothpick comes out with a few moist crumbs.
Cool. Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to finish cooling.
Warm is great; piping hot can make them gummy.