Whisk together the flour, cocoa powder, baking powder, and baking soda in a large bowl until combined. Make a well in the center of the flour mixture and add the milk, sugar, espresso powder, and oil. Whisk the ingredients together until blended.
Preheat a waffle iron for 4 minutes, or according to the manufacturer's instructions. (A drop of water should sizzle and bounce when dropped on the iron.) Meanwhile, beat the egg whites and salt with an electric mixer at high speed just until they form soft peaks. Fold the whites into the chocolate batter in 3 additions, folding in the chocolate chips with the last addition of whites. Fold just until the mixture is combined.
Coat the heated waffle grids with cooking spray right before using. Add enough batter to almost cover the waffle grids (3/4 cup) and cook for 3 to 4 minutes. Repeat with the remaining batter. (To keep warm, place a single layer of waffles on a foil-lined baking sheet in a preheated 250°F oven.) Serve with maple syrup.
Tip: With a good amount of fiber, these chocolaty treats will keep you satisfied for hours.