Preheat the oven to 375F and position the top rack in the middle of the oven. Line a 12-cup muffin pan with paper liners. You can also grease generously with oil.
Mash the bananas until they’re almost smooth. Melt the butter then let it sit on the counter to cool down to room temperature.
Make the batter:
In a large bowl, use a wire whisk to mix the flour, oats, sugar, baking powder, baking soda, cinnamon, and salt.
In a medium bowl, whisk the eggs with a fork. Add the mashed bananas, milk, melted butter, and vanilla and mix to combine the liquid mixture.
Pour the liquid mixture into the dry ingredients bowl and with the whisk, mix to combine them into a thick batter. Add the chocolate chips and mix them into the batter.
Fill the lined muffin cups with ¼ cup of batter each (nearly to the top). Sprinkle the tops with a few oats, if you want.
Bake:
Bake the muffins in a preheated oven for 22 to 25 minutes, until the tops are golden and the centers are set. They’ll feel firm to the touch and bounce back.
Remove the muffin pan from the oven, let them cool in place for 10 minutes, then remove them from the pan and enjoy.
Storage:
Leftover muffins can be stored in an airtight container on the counter for 2 days, in the fridge for up to a week. They can be frozen for up to 3 months. To enjoy, reheat them for 20 to 30 seconds in the microwave.