Preheat oven to 350° F. Line a muffin tin with cupcake liners or parchment baking cups.
In a large bowl, whisk together sugar, egg, vegetable oil, milk, and vanilla until the mixture is combined and smooth. Stir in the mashed bananas and oats.
In a separate bowl, combine the flour, baking soda, baking powder, and salt; mix until combined. Add the dry ingredients into the large bowl of wet ingredients and stir just until combined. Fold in the walnuts and dark chocolate chips. DO NOT OVER MIX.
Scoop batter into the lined muffin cups so they are about ⅔ of the way full. Top the batter with a walnut half, dark chocolate chips, and/or sprinkle with plain oats. Bake for about 18 – 20 minutes, or until golden brown and a toothpick inserted into the center of a muffin comes out clean. Remove muffins from the pan and place on a cooling rack to cool.