Preheat oven to 400 degrees F. Grease your 12-cup standard muffin tin or your 24-cup mini muffin tin.
In a medium bowl combine flour, sugar, oats, baking soda, baking powder, and salt. Set aside.
In a large bowl beat the egg with a fork. Stir in the milk, vanilla extract, and oil. Place peeled ripe bananas on a large plate and using a fork mash until smooth. Add this to the egg mixture and stir to combine.
Using a wooden spoon, gently fold in the flour mixture into the banana mixture. Once combined, gently fold in the chocolate and chopped nuts. (optional: reserve some to top the muffin batter)
If making standard size muffins, divide the batter evenly among the tin, (optional: top each muffin with a sprinkle of oats, nuts, and chocolate), and bake at 400 degrees F for 15-20 minutes until toothpick comes out clean.
If making mini muffins, fill the batter right to the tops. (optional: top each muffin with a sprinkle of oats, nuts, and chocolate), and bake at 400 degrees F for 10 minutes until toothpick comes out clean.
Set on cooling rack and enjoy!
Storage: After completely cooled down to room temperature, store in ziplock bags or sealed tupperware container on counter up to 4 days.
Freezing: These muffins freeze really well! After they're completely cooled down to room temperature, wrap each muffin in plastic wrap, place in ziplock bag or tupperware container and place in freezer for up to 3 months.
Thawing: These can be slowly thawed at room temperature, or microwaved unwrapped, or wrapped in foil in the oven and baked at 350 degrees F for 10-15 minutes.