Prepare Your Oven and Muffin Pan: Start by preheating your oven to 425°F (218°C). Line a 12-cup muffin tin with paper liners or lightly grease the cups with nonstick spray to prevent sticking.
Mix Dry Ingredients: In a medium mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside — this will be your dry mixture.
Combine Wet Ingredients: In a separate large bowl, mash the ripe bananas thoroughly. Add the melted butter, brown sugar, egg, vanilla extract, and milk. Whisk or beat with an electric mixer on medium speed until everything is fully blended.
Bring It All Together: Gradually add the dry mixture to the wet ingredients. Stir just until combined, being careful not to overmix. If you're including nuts or chocolate chips, gently fold them in at this point. The batter should be thick.
Fill the Muffin Cups: Divide the batter evenly among the muffin cups, filling each one to the top. This helps achieve tall, bakery-style muffins.
Bake at Two Temperatures: Bake in the preheated oven at 425°F (218°C) for 5 minutes. Then, without opening the oven door, reduce the heat to 350°F (177°C) and continue baking for 16–18 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Total baking time is about 21–23 minutes.
Cool Before Serving: Allow the muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.