Preheat the waffle iron. Aim for medium to medium-high heat.
Lightly grease it so nothing sticks.
Mix the dry ingredients. In a bowl, whisk oat flour, protein powder, cocoa, baking powder, and salt until no clumps remain.
Whisk the wet ingredients. In another bowl, combine egg, egg whites, Greek yogurt, milk, and vanilla. Whisk until smooth.
Combine. Pour wet into dry and stir just until combined. The batter should be thick but pourable.
Add a splash of milk if it’s too stiff.
Add Nutella. Warm Nutella 10–15 seconds in the microwave. Fold 1–2 tbsp into the batter for overall flavor. Reserve a little to swirl on top of each waffle if you like.
Cook. Pour batter onto the hot iron (don’t overfill).
If swirling, add 1 tsp Nutella on top and marble it with a toothpick. Close and cook until steam slows and the waffle is golden and crisp, 3–5 minutes depending on your iron.
Repeat. Keep cooked waffles on a wire rack so they stay crisp. Finish remaining batter.
Serve. Top with fruit, a small drizzle of Nutella, and chopped hazelnuts for crunch.
Add a spoon of yogurt if you want extra protein.