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).