Position an oven rack in the center position of the oven. Heat the oven to 350℉. Line a 24 cup mini muffin pan with paper mini muffin liners. Set aside.
Heat a nonstick skillet over medium heat. Cut a medium banana into 24 rounds that are each ¼" thick. Melt the butter in the heated skillet, swirling to coat the bottom. Lay each banana round in the skillet in one even layer. Cook undisturbed for 2 minutes, letting the bananas brown and caramelize. Carefully flip each banana round, then brown and caramelize the other side for another 2 minutes. Remove from the skillet and place on a plate to cool.
1 medium banana, 1 tsp butter
Make muffin batter
In a medium bowl, whisk the flour, cinnamon, baking powder, baking soda, and salt together.
1 ½ cups whole wheat flour, ½ tsp cinnamon, 1 ½ tsp baking powder, ¼ tsp baking soda, ½ tsp salt
In a large bowl, mash the remaining banana (not the ones you caramelized). Add the oil, maple syrup, egg, and vanilla to the mashed bananas. Whisk until combined.
⅔ cup mashed banana, ⅓ cup neutral oil, ¼ cup maple syrup, 1 whole egg, 1 tsp vanilla
Pour the dry ingredients in with the wet ingredients and mix until all the flour is hydrated.
Top muffins and bake
Portion the batter among the muffin pan. Add a caramelized banana topper to the top of each muffin, very slightly pressing it into the center of the top of the muffin.
Bake for 15 minutes, then remove the muffins from the oven and let cool completely on a wire cooling rack before enjoying.