Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with liners.
In a bowl whisk together flour, baking powder, baking soda, and salt until combined.
In a second large bowl mash the ripe bananas until mostly smooth.
Add sugar to the mashed bananas and stir to combine.
Beat in the egg, then add the oil (or melted butter), milk, and vanilla; mix until smooth.
Pour the wet mixture into the dry ingredients and fold gently with a spatula until just combined; do not overmix.
Fold in optional mix-ins like chopped walnuts or chocolate chips if using.
Divide batter evenly among the 12 muffin cups, filling about two-thirds full.
Bake in the preheated oven for 18–22 minutes or until a toothpick inserted in the center comes out clean.
Cool muffins in the tin 5 minutes, then transfer to a wire rack to cool completely before serving.