Dark Chocolate Waffles

• 2 cups all-purpose flour
• ½ cup unsweetened cocoa powder
• ¼ cup (packed) brown sugar
• 2 teaspoons baking powder
• 1 teaspoon baking soda
• 1 teaspoon kosher salt
• 3 large eggs, separated
• 2 cups buttermilk
• ½ cup olive oil
• 1 teaspoon vanilla extract
• 6 oz. bittersweet chocolate (at least 70% cacao), finely chopped

Nonstick vegetable oil spray

• Unsalted butter and warm pure maple syrup (for serving)

1. Preheat oven to 250°. Whisk flour, cocoa powder, brown sugar, baking powder, baking soda, and salt in a large bowl. Make a well in the center and add egg yolks, buttermilk, oil, and vanilla. Blend with a fork, then gradually incorporate dry ingredients, mixing just until combined.
2. Using an electric mixer on medium-high speed, beat egg whites in a small bowl until soft peaks form. Working in 2 batches, fold egg whites into batter just until combined. Fold in chocolate.
3. Heat a waffle iron until very hot; lightly coat with nonstick spray. Working in batches, cook waffles until cooked through. Transfer to a wire rack set inside a baking sheet, cover loosely with foil, and keep warm in oven until ready to serve.
4. Serve waffles with butter and syrup.
5. Do Ahead: Batter with egg whites can be made 4 hours ahead. Cover and chill.