Step 1: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Whisk thoroughly to ensure ingredients are evenly distributed.
Step 2: In a separate medium bowl, whisk together the buttermilk, melted butter, and vanilla extract. Add the egg yolks to the wet ingredients and whisk until well combined.
Step 3: Pour the wet ingredients into the dry ingredients. Mix gently with a whisk or wooden spoon until just combined. A few lumps are fine; do not overmix.
Step 4: In a clean, separate bowl, beat the reserved egg whites with an electric mixer until stiff peaks form.
Step 5: Gently fold the whipped egg whites into the waffle batter in two additions, using a spatula and a lifting motion. Be careful not to deflate the egg whites.
Step 6: If time allows, let the batter rest for 5-10 minutes. Meanwhile, preheat your waffle iron according to the manufacturer's instructions. Lightly grease if not non-stick.
Step 7: Pour or scoop the batter onto the hot waffle iron, filling it about two-thirds full. Close the lid and cook for 3-5 minutes, or until golden brown and steam is no longer escaping.
Step 8: Gently remove the cooked waffles and serve immediately with your favorite toppings. For larger batches, keep warm in a 200°F (90°C) oven on a wire rack.