Preheat the oven to 350°F. Prepare a muffin pan by greasing it with cooking spray or inserting cupcake liners.: The scent of the warm oven signals that the batter will meet heat in a predictable environment, helping the muffins rise steadily. I always give the oven at least ten minutes to reach temperature so the first cracks on top form correctly and the interior cooks through. You should hear the faint hum of the oven and notice the heat radiating when you open it briefly. A common pitfall is skipping preheating, which can result in uneven rise and dense centers.
In a bowl, sift flour, baking powder, baking soda, and salt.: Sifting aerates the all purpose flour and evenly distributes the leaveners, producing a lighter crumb and reducing lumps. You will see a lighter, fluffy pile after sifting, and the fragrant hint of salt will be more subtle. The sound is a soft whisper as the dry ingredients fall through the sifter. If you skip this, you might get pockets of baking soda that cause uneven flavor or odd texture.
In a separate bowl, mix together bananas, sugar, egg, and melted butter. Fold in flour mixture, and stir until smooth. Scoop batter into greased muffin cups. Sprinkle with cinnamon and sugar.: The mashed bananas should be silky, almost jammy, and when you whisk them with sugar , egg , and slightly cooled butter you get a glossy, fragrant wet mix. Folding the dry mixture in should be gentle and deliberate, with a few streaks of flour disappearing to avoid overworking gluten, which keeps the crumb tender. As you scoop batter, watch for a smooth, slightly thick texture that holds shape but still settles a bit when tapped. When you dust the tops with the extra sugar and cinnamon , they glint and promise a caramelized crust. A typical mistake here is overmixing, which yields tunnels and tough muffins.
Bake for 25 to 30 minutes. Bake mini muffins for 10 to 15 minutes. Enjoy!: As the muffins bake, the kitchen fills with warm banana and cinnamon notes, and the tops start to take on a golden tan. The edges will pull slightly away from the pan and a toothpick inserted in the center should come out clean or with a few moist crumbs. The tops should feel springy to the touch and you may hear a faint crack when the sugar caramelizes. For mini muffins, the shorter time prevents drying out. A common issue is opening the oven too often, which drops temperature and can lead to uneven crumb or collapse, so trust the stated times and check only near the end.