Stir the yeast through the lukewarm water. Set aside in a warm spot for now.
Cut the butter into cubes. Add the butter to the milk and warm both on a low heat. Don’t let the milk come to a boil! Warm the milk until the butter has melted, then take the pan from the heat and set aside to cool a bit.
Separate the eggs and beat the egg whites into stiff peaks.
In a large bowl, mix the salt with the flour. Add the dissolved yeast, egg yolks and sugar and vanilla sugar. Mix well.
Bit by bit, add the warm milk. Whisk thoroughly to get rid of any lumps.
Add about 1/3 of the egg whites to the batter. Fold it in carefully, to avoid knocking out all of the air. Then carefully fold in the rest of the egg whites, until everything is incorporated. The batter should be smooth without lumps or large pieces of egg white.
Then cover the bowl with cling film and leave in a warm (not hot) place. Let the batter prove for about 30 minutes.
Preheat the waffle iron before brushing on a bit of oil.
Scoop a bit of batter onto hot waffle iron, then cook for a couple of minutes until golden brown.
Serve the waffles warm, with your favourite toppings.
How many waffles you’ll be able to make from this batter, of course depends on the size of your waffle iron. We managed to make about 20, so we had more than enough! Luckily, these waffles are still tasty when eaten cold the next day.