Preheat the oven to 350°F (177°C) and line a mini muffin pan with paper liners. Whisk the flour, baking powder, baking soda, salt, and cinnamon in a small mixing bowl. Set aside.

Next, peel the bananas. Place them in a shallow bowl and use the tines of a fork or a potato masher to smash the bananas into a very smooth purée. You should have about 325-350 grams or 1 1/2-1 3/4 cups of mashed banana.

Add the melted and cooled butter, granulated sugar, dark brown sugar, vanilla extract, and sour cream to a large mixing bowl. Whisk until smooth. Then, mix in the mashed banana. The mixture may curdle a little, but it'll smooth out!

Pour the flour mixture into the wet ingredients and mix until the batter is about 75% combined. Then, add the toasted and chopped pecans or walnuts. Fold the nuts into the batter just until the batter is well-mixed and there are no streaks of flour. There may be a few lumps, but that's ok! Just don't over-mix the batter or the muffins will come out dense.

If you have time, cover the bowl and let the batter sit for 30 minutes, until it's nice and fluffy. This step is optional, but gives the baking powder a head start which makes the muffins extra tall.

Use a scoop to divide the batter between the mini muffin liners, filling them almost all the way to the top. Sprinkle the tops of the muffins with extra chopped pecans or walnuts, if desired.

Bake the muffins for 11-12 minutes, just until an inserted toothpick has a few moist crumbs. Let the muffins cool in the pan for 5 minutes and then transfer them to a wire rack to cool completely. Enjoy!