Preheat the oven to 350ºF. Prepare a 12-cup muffin tin or 6-cup jumbo muffin tin with liners or spray it with baking spray.

Whisk the flour, baking soda, cinnamon, nutmeg, and salt together in a medium size bowl. Set aside.

Place mashed bananas and sugar in a large bowl. Use a whisk or hand-held beater to combine. Add the egg, buttermilk, melted butter, and vanilla, and beat just until combined.

Fold in the flour mixture with a wooden spoon or spatula. Fill the muffin tins just over halfway full or until the batter is evenly distributed.

Bake the muffins for 15-20 minutes, or until a toothpick inserted into the center comes out clean. Let cool on a wire rack. Serve warm or at room temperature. Store in an airtight container for up to 3 days or freeze for 2 months.