Combine first 5 ingredients in a medium bowl.
Combine buttermilk, peanut butter, maple syrup, oil, and vanilla in a medium bowl.
Separate eggs and add egg yolk to buttermilk mixture.
Whip egg whites to stiff peaks with an electric mixer.
Add buttermilk mixture to flour mixture and stir until combined. Add egg whites and fold gently into batter. Stir in finely chopped dark chocolate.
Heat an electric waffler according to manufacturer's instructions. Cook waffles, keeping cooked waffles warm in a warm oven.