Preheat your oven to 350°F (175°C) to ensure even baking and proper rising.
Line a 12-cup muffin tin with paper liners or lightly grease it with oil to prevent sticking.
In a large mixing bowl, whisk together the whole wheat flour, baking powder, baking soda, and salt to evenly distribute the leavening agents.
In another bowl, combine the mashed bananas, eggs, avocado oil, brown sugar, buttermilk, and vanilla extract. Whisk until smooth and well incorporated.
Gradually add the wet ingredients to the dry ingredients, stirring gently with a spatula. Mix just until combined; some lumps are fine.
Gently fold in two-thirds of the Nutella into the batter to create delicious chocolatey swirls.
Spoon the batter into the prepared muffin tin, filling each cup about two-thirds full. Add a dollop of the remaining Nutella on top of each muffin for extra indulgence.
In a small bowl, mix together the brown sugar, flour, cinnamon, and softened butter until crumbly. Sprinkle this crumb topping generously on each muffin.
Bake the muffins for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Remove the muffins from the oven and let them cool in the pan for 5 minutes before transferring to a cooling rack to cool completely. Enjoy warm or at room temperature!