Batter must be made 12 - 24 hr in advance. BATTER MADE DAY BEFORE COOKING.
Heat milk and butter in small saucepan until butter is melted, 3-5 min. Cool milk/butter mixture until warm to touch.
Meanwhile whisk flour, sugar, salt, and yeast in medium dry bowl. Gradually whisk warm milk/butter mixture into flour mixture, whisking until batter is smooth.
Whisk egg and vanilla until combined. Add egg mixture to batter and whisk until incorporated. Scrape down sides of bowl, cover with plastic wrap, refrigerate at least 12 and up to 24 hr.
Prior to making waffles whisk to recombine. Bake according to waffle maker instructions.