Special equipment: Waffle iron
Preheat the waffle iron to the regular setting.
Sift together the flour, sugar, baking powder and salt in a bowl. In a separate bowl, whisk together the milk, vanilla and 2 egg yolks. Pour over the dry ingredients and very gently stir until halfway combined. Pour in the melted butter and continue mixing very gently until combined.
In a separate bowl using a whisk (or a mixer), beat the 4 egg whites until stiff. Slowly fold them into the batter, stopping short of mixing them all the way through.
Scoop the batter into your waffle iron in batches and cook according to its directions (lean toward the waffles being a little deep golden and crisp!). Serve immediately with softened butter and warm syrup.