Preheat a waffle iron. Place oats in the bowl of a food processor, and process until coarsely ground, about 30 seconds. Transfer to a medium bowl, and whisk in whole-wheat flour, cake flour, dry milk, sugar, salt, baking powder, and baking soda.
In another medium bowl, whisk together egg, egg whites, and buttermilk. Pour into the dry ingredients, add melted butter, and whisk together until thoroughly combined.
Spray waffle iron with cooking spray. Quickly pour about 1 1/2 cups batter onto center of grid. Use a rubber spatula to spread the batter evenly out to the edges. Bake waffles until golden brown, about 5 minutes. Transfer to a warm oven. Repeat until all batter is used.
Cut waffles into squares; serve two per person topped with mango sauce and fruit.