In a small bowl, sprinkle the yeast over the lukewarm water and let sit until foamy, about 5 minutes.
In the large bowl of a stand mixer, mix the flour and salt. Pour in the yeast mixture and gradually add the eggs while beating. Beat in the vanilla using the paddle attachment.
While the mixer is on medium speed, gradually beat in the melted butter. Beat until smooth and the butter is incorporated. The batter will be thick and sticky.
Cover the bowl with a damp towel and set aside at room temperature until doubled in size, about 1 hour and 45 minutes.
Meanwhile, crush the sugar cubes into small pieces (but not powder!) using a rolling pin. Mix the crushed sugar into the risen batter. Cover again and lest rest for 15 minutes.
Heat the waffle iron. Once hot, place about 2 Tbsp of batter for each waffle, close, and let bake. As sugar melts at a lower temperature than a regular waffle iron, you may need to monitor the iron and unplug it half-way through baking to prevent the sugar from burning. Keep the waffle still in the hot iron, though, so that it can still bake.
Once cooked ,transfer the waffles to a plate. Serve with whipped cream!