In a small bowl, dissolve yeast in 1/4 cup warm milk. Let stand for about 10 minutes.
In a large bowl, whisk together the egg yolks, 1/4 cup of the warm milk and the melted butter. Mix in the yeast mixture, sugar, syrup, salt and vanilla. Stir in the remaining 2 1/2 cups milk alternately with the flour. Cover the bowl tightly with plastic wrap.
Make ahead: place the batter and covered egg whites in the fridge overnight. Remove from the fridge about 30 minutes before baking the waffles.
If baking right away, let the batter rise in a warm place until doubled in volume, about 1 hour.
Beat the egg whites until they form soft peaks; fold into the batter.
Preheat the waffle iron. Spray with oil and spoon about 1/2 cup onto center of iron. Close the lid and bake until it stops steaming and the waffle is golden brown. Serve immediately or keep warm in 200 degree oven.