Preheat oven to 400 degrees. Line a 12-muffin tray with paper liners. Set aside.
In a standing mixer, combine both types of flour, rolled oats, cinnamon, baking powder, baking soda, salt, and mashed banana and blend on low speed for 2 minutes. Add the walnuts, brown sugar, oil, eggs, and buttermilk and blend for 2 minutes on medium speed. Add the vanilla and mix until well combined.
Using an ice cream scoop, portion the batter evenly into the muffin tin and top each muffin with 3-4 chocolate chips. Bake for 15 minutes. Once baked through, remove from oven and let cool.