Preheat your oven to 400°F and line a standard 12-cup muffin pan with paper liners.
Combine flour, baking powder, baking soda, salt and nutmeg in a medium bowl and whisk to blend evenly.
Mash the bananas well with a fork until smooth and add them to a large bowl. Whisk in sugar, egg, yogurt and vanilla. Add melted butter and milk and whisk until combined. Stir in the oats and set the mixture aside for 5-10 minutes to let the oats hydrate.
Add the flour mixture to the banana mixture and fold it in gently until just combined. Do not over-mix.
Divide batter evenly among prepared muffin cups, filling them right up to the rim. Scatter some chocolate chunks on top or sprinkle some oats on top and bake for 5 minutes. Lower the oven temperature to 375°F and bake for another 10-15 minutes until golden brown and firm to the touch. Transfer pan to a wire rack and let muffins cool for 5 minutes in the pan before transferring individually to the rack to finish cooling.