Preheat the oven to 425°F (218°C). Grease a 12-count muffin pan with oil or butter or line with cupcake liners.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
In a large bowl (or in the bowl of a stand mixer), mash the bananas until smooth. Add in the avocado oil, coconut sugar, egg, vanilla extract, and milk. Mix on medium speed until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. The batter will be thick. If you're adding chopped nuts or chocolate chips, gently fold them into the batter now.
Divide the batter evenly among the prepared muffin cups, filling each to the top.
Bake at 425°F for 5 minutes, then reduce the oven temperature to 350°F (177°C) without removing the muffins. Continue baking for an additional 16–18 minutes, or until a toothpick inserted in the center comes out clean (12–14 minutes for mini muffins).
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.