1. Preheat oven to 375°F and line a muffin tin with liners or grease it.
2. Mash 2-3 very ripe bananas with a fork or potato masher in a bowl.
3. In a separate large bowl, whisk together flour, sugar, baking powder, baking soda, salt, cinnamon, and nutmeg.
4. In another bowl, mix mashed bananas, melted butter, egg, vanilla, and buttermilk until well combined.
5. Pour wet ingredients into the dry ingredients and gently mix until just combined. Avoid overmixing.
6. If using mix-ins like chocolate chips or nuts, fold them in gently.
7. Fill muffin cups about 3/4 full with the batter.
8. Optionally, sprinkle with streusel topping for extra flavor and texture.
9. Bake for 5 minutes at 400°F, then reduce temperature to 375°F and continue baking for 10-15 minutes until a toothpick inserted in the center comes out clean.
10. Remove muffins from the oven and let them cool in the pan for a few minutes.
11. Transfer muffins to a wire rack to cool completely before storing.
12. Store at room temperature for 2-3 days or freeze for up to 6 months.