Preheat oven to 250°F and place a wire rack over a baking sheet, set aside.

In a large bowl, combine the flour, cocoa, brown sugar, baking powder, baking soda and salt. Whisk until evenly combined. Make a well in the center of the bowl and pour in the egg yolks, buttermilk, coconut oil, and vanilla extract. Mix until just combined and lump-free.

In a separate bowl, beat the egg whites until stiff peaks form. Make sure the bowl and beaters are super clean so that the egg whites fluff up properly. Add the beaten egg whites to the batter, a little at a time, and fold them in gently until evenly combined. Fold in the shredded zucchini and chopped dark chocolate.

Heat waffle iron and lightly grease with cooking spray. Add about ¼ cup of batter to the center of the hot waffle iron. Close lid and cook according to your waffle iron's instructions. Transfer to prepared wire rack baking sheet. Keep waffles warm in the preheated oven while you continue to cook the rest. Serve with butter and maple syrup or with a scoop of vanilla ice cream. Enjoy!