Preheat oven to 325°F. Grease or line a 12 cup muffin tin.
In a large bowl, whisk together flours, oats, baking soda, salt, and cinnamon.
In a medium bowl, whisk together oil and honey or maple syrup. Add the mashed banana, eggs, milk, and vanilla and whisk together.
Pour the wet ingredients into the bowl of dry ingredients and fold together, stirring gently until no more flour is visible and ingredients are just combined.
Fill muffin tins half full (a cookie scoop is really handy here). Using two spoons, drop about 1 teaspoon of Nutella in the centre of the batter in each tin. Top up the muffin tins with the remaining batter, covering up the Nutella centre. Warm ¼ cup of Nutella in the microwave for 30 seconds, or just enough to thin it out a bit. Drop or drizzle about ½-1 teaspoon of Nutella on the tops of the muffins. Using a knife or a toothpick, swirl it into the top layer of batter but try not to disrupt the Nutella centre. Don't worry if it doesn't look perfectly swirled. Sprinkle a few chocolate chips on the tops, if desired.
Bake muffins for 22-24 minutes or until your finger can gently press the tops without leaving an indent (test where there is just batter and not Nutella). Let muffins cool for 5 minutes in the pan, then remove and let cool completely on wire racks.