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!