Place the milk in a medium bowl. Add vinegar to the milk and set aside to let it sour a little.
In a small bowl mix flour, baking powder, baking soda, vanilla sugar, and granulated sugar. If you are using vanilla extract, you can add it to the wet ingredients.
Separate the egg yolks from the whites. Keep the whites in a separate small container and add the yolks and melted butter into the sour milk. Give the wet mixture a stir and slowly add dry ingredients into it. The batter is going to be somewhat thicker. Set aside until you whip the egg whites up.
Add lemon juice to the egg whites and whisk with a hand-held mixer. Whisk until soft peaks form and hold the foam can be held on the tip of the whisk without sliding. It should take about 2 minutes.
Slowly fold in egg whites into the batter. Make sure not to over-mix things.
Heat your waffle maker according to the manufacturer’s guidelines. If you know your waffle maker well, it shouldn’t be a problem to pinpoint which temperature works well for waffles.
Spray or brush a little oil on the waffle maker. Using a measuring cup which comes with your waffle maker, pour a little of the batter in the center of the waffle iron. The machine should beep when the waffle is ready. If you do not have a measuring cup which comes with the waffle maker, aim at adding around 1/4 cup full of the batter in the waffle iron.