Preheat your Belgian waffle maker according to manufacturer's instructions. In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined.

In a separate medium bowl, whisk together the egg yolks, warm milk, melted butter, and vanilla extract until smooth. Gently warm the Nutella in the microwave for 15-20 seconds until pourable, then whisk it into the wet ingredients.

In a clean bowl using an electric mixer, beat the egg whites on medium-high speed until stiff peaks form, about 2-3 minutes.

Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined. Some lumps are fine — do not overmix.

Fold the beaten egg whites into the batter in two additions, using gentle sweeping motions to maintain airiness. Let the batter rest for 5-10 minutes.

Lightly grease the preheated waffle iron with cooking spray. Pour the recommended amount of batter for your waffle maker onto the center of the iron, then drizzle about 1 tablespoon of Nutella over the batter in a swirl pattern.

Close the lid and cook until the waffle is golden brown and steam stops escaping, typically 4-5 minutes depending on your machine. Avoid opening early to prevent sticking.

Carefully remove the waffle with a fork or silicone tongs and place on a wire rack. Repeat with remaining batter, re-greasing the iron between batches.

Serve immediately while warm and crispy, or keep finished waffles warm in a 200°F oven on a baking sheet until ready to serve.