Preheat the waffle iron to high heat.
In a large bowl, combine flour, cocoa powder, brown sugar, baking powder, baking soda, and salt.
Separate the eggs into yolks and whites. Beat the egg whites until stiff peaks form.
Mix the egg yolks, buttermilk, oil, butter, and vanilla extract into the dry ingredients. Gently fold in the stiff egg whites until just combined. Gently fold in the bittersweet chocolate.
Brush the heated waffle iron with oil. Add batter to the waffle iron, amount based on size of iron. Heat until cooked through and fluffy. Transfer to plate and repeat with remaining batter.
Serve immediately topped with whipped cream, strawberry preserves, cacao nibs, or your favorite toppings.