1. Place the butter in the bowl fitted with the ultrablade knife and melt at speed 5 at 190°F for 3 minutes.
2. Add the milk, egg yolks and sugar and mix at speed 10. Gradually add the flour through the opening. Once the flour is mixed in, add the vanilla and mix for another 2 minutes. Pour the mixture into a separate bowl and wash the Prep&Cook bowl.
3. Place the egg whites and salt in the bowl fitted with the whisk. Mix at speed 6 for 5 minutes, without the stopper.
4. Using a spatula, gently fold the whipped egg whites into the flour mixture.
5. Heat the waffle maker and lightly grease with butter. When it is hot, pour a ladleful of batter into waffle maker and cook as directed by manufacturer. Repeat until all batter is used.