Set your oven to 425°F (218°C) and grease a 12-cup muffin tin or fit it with paper baking liners. Preparing the pan ahead ensures the batter can go straight into the oven as soon as the leaveners are mixed.

Whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl until thoroughly blended, then set it aside. Evenly dispersing the dry ingredients prevents clumps of leavening from creating bitter spots in the crumb.

Mash the ripe fruit in a separate large mixing bowl, then whisk in the melted butter, brown sugar, room-temperature egg, vanilla, and milk until smooth. The wet mixture should look glossy, uniform, and slightly thickened with small fruit flecks.

Pour the dry ingredients into the wet mixture and stir gently with a spatula just until the flour streaks vanish, folding in any optional nuts or chocolate chips at the very end. The finished batter will appear thick, rustic, and slightly lumpy rather than silky smooth.

Divide the batter evenly among the muffin cups, filling each liner all the way to the top. Filling the cups completely gives the batter the volume needed to push upward into rounded crowns.

Bake at 425°F (218°C) for 5 minutes, then, without opening the oven door, lower the heat to 350°F (177°C) and bake for 16 to 18 minutes longer until a wooden toothpick inserted into the center comes out clean. A sweet, buttery cinnamon aroma fills the kitchen around the fifteen-minute mark as the tops turn golden brown.

Rest the muffins in the pan for 5 minutes before lifting them onto a wire rack to finish cooling. This short rest stabilizes the crumb while moving them to the rack stops steam from softening the bottoms.