The night before…
Melt the butter in a covered saucepan over medium-low heat. Of course you can also use a microwave if you prefer– I just don’t happen to have one. Let the butter cool for ten minutes or so. Meanwhile, combine the flour, sugar, salt, and yeast in a mixing bowl and stir it all up with a fork. Once the butter’s cooled off, pour it into the bowl of dry ingredients, then add the milk. Stir until the liquid ingredients have been well-incorporated into the mix, and you have a reasonably smooth-looking batter. Cover the bowl with plastic wrap, and let it sit at room temperature overnight.

When you’re ready to eat …
Saunter into your kitchen and take a peek at your batter. A miraculous transformation will have taken place: instead of the smooth, normal-looking waffle batter you had the night before, you should now see a puffy, porous mixture that’s substantially more voluminous. It may also have an odd, vaguely sourdough-like, yeasty smell. All this is good.

Pull out your waffle maker and plug it in. Spray or brush it lightly with cooking oil. While it’s heating up, you can finish up the batter. Separate the egg white into a small mixing bowl, and drop the yolk into the yeast batter. Stir in the egg yolk. In the small bowl, beat the egg white until it can hold nice, stiff peaks. Gently fold the beaten egg white into the batter.

Time to start waffling! Ladle the batter into the waffle maker and bake for 3-5 minutes, until deep golden brown. Serve immediately.