A little bit of planning before bedtime means you get tasty waffles in the morning.
Place milk and butter in medium microwave-safe bowl. Cover bowl and heat in microwave for 1 minute. Stir mixture with rubber spatula. Continue to heat in microwave until butter is melted and milk is warm, 1 to 2 minutes. Let milk mixture cool, uncovered, until just warm, about 5 minutes.
In large bowl, whisk together flour, sugar, yeast, and salt.
In small bowl, whisk eggs and vanilla until combined. Slowly whisk warm milk mixture into flour mixture until smooth, then whisk in egg mixture.
Scrape down bowl with rubber spatula. Cover bowl tightly with plastic wrap and refrigerate for at least 12 hours or up to 24 hours.
Heat waffle iron. When waffle iron is hot, remove batter from refrigerator and discard plastic. Whisk batter to recombine (batter will deflate).
Use dry measuring cup to pour batter into middle of waffle iron. (Use about ½ cup batter for 7-inch round waffle iron or about 1 cup batter for 9-inch square waffle iron.) Close waffle iron and cook until waffle is golden brown.
Overnight waffles (also known as raised or yeasted waffles) are very old-school. They do require advance planning, but they are the crispiest, tastiest waffles on the planet. They are very light and airy because they contain yeast—the magic ingredient that makes bread rise. Make the batter before bed (or even earlier in the day). When you wake up the next morning, all you have to do is heat the waffle iron, pour in the batter, and enjoy.
You can freeze any leftover waffles for one month. Stack cooled waffles with parchment or wax paper between them and then put the stack of waffles in a zipper-lock bag. Heat waffles one at a time in toaster on its lowest setting until warmed through and crisp on the outside.
When making waffles, pancakes, or french toast for a crowd, it can be nice to serve everything at once. If you don’t, you end up being the cook while everyone else eats. To keep early batches warm, preheat your oven to 200 degrees—just warm enough to keep them hot but not so warm that they will dry out. As you make the waffles (or pancakes or french toast), place them on a cooling rack set in a rimmed baking sheet. Then place the baking sheet in the warm oven. Waffles, pancakes, and french toast can stay in the oven for 15 minutes—long enough for you to cook the remaining batches.