Preheat oven to 350 degrees F. Line muffin pans with paper liners or coat with cooking spray. Set aside.
To make the crumble. Add the flour, brown sugar, quinoa, and cinnamon in a medium bowl and mix until combined. Rub in the butter with your fingers until the mixture is crumbly. Stir in the walnuts. Set aside.
In a large bowl, whisk together whole wheat flour, all-purpose flour, brown sugar, cocoa powder, lemon zest, cinnamon, baking powder, baking soda and salt. Stir in the cooked quinoa, making sure it's completely incorporated with the flour mixture. Set aside.
In a medium bowl, whisk together the egg, buttermilk, vanilla, coconut oil, and mashed banana. Add the wet ingredients to the dry ingredients and stir until just combined. Gently stir in chocolate chips (chunks) and diced banana.
Spoon muffin mixture into prepared pan, filling each cup 3/4 of the way full. Sprinkle each muffin with crumble topping. Bake for about 25 minutes, until a toothpick inserted in the center of a muffin comes out clean. Let cool in pan for 5 minutes before removing to a wire rack to cool completely. Enjoy!