Whisk together 1 cup all-purpose flour, 1 cup whole wheat flour, 1/2 cup cocoa powder, the coarsely chopped hazelnuts, 1/4 cup chocolate chips, 1 Tablespoon baking powder, 1 teaspoon baking soda, and 1/4 teaspoon table salt in a large bowl. Set aside.
In a smaller bowl, blend (using an immersion blender is easiest) or stir together 1 1/2 cups mashed banana, 1/2 cup granulated sugar, 1/2 cup buttermilk, 1 large egg, and 1 teaspoon vanilla extract. Gently fold the banana mixture into the flour mixture just until no more powdery bits remain. Leave the batter to rest at room temperature for about 15 minutes while you heat the oven (or cover the batter and place it in the fridge if you'd like to let it rest longer.)
Heat the oven to 400°F. Prepare a muffin pan by lining the cups with paper liners or greasing them with neutral-flavored cooking oil. Once the oven is at temperature and the batter has rested, use a large spoon to fill the muffin cups most of the way full. Sprinkle the top of the batter with a little extra chopped hazelnuts and/or chocolate chips, if desired (or you could top them with coarse sugar if you like).
Bake for 15-18 minutes until a toothpick inserted into the center of the muffins comes out with a few crumbs clinging to it. Cool muffins for at least 10 minutes before serving. To store leftover muffins, cool them to room temperature and store them in a zipper bag or airtight container at room temperature for up to 2 days, or in the freezer for longer.
Notes
For convenience, consider using buttermilk powder (follow the directions on the label). Alternatively, you can use soured milk (1 Tablespoon of lemon juice or vinegar and enough milk to add up to 1/2 cup), plain or vanilla yogurt, or a 1:1 mixture of plain or vanilla Greek yogurt and milk. Use a dairy-free yogurt alternative to make these muffins dairy-free (you'll also need to make sure to use dairy-free chocolate chips).