In a large bowl, combine the warm milk and yeast. Let stand for 5 minutes until foamy. Whisk in the sugar, eggs, melted butter, and vanilla extract until smooth.
Add the flour and salt to the wet ingredients. Whisk until just combined with no large lumps remaining. The batter will be thinner than pancake batter.
Cover the bowl tightly with plastic wrap and refrigerate overnight, or let rise at room temperature for at least 2 hours until visibly puffy and bubbly.
When ready to cook, preheat your Belgian waffle iron according to manufacturer's instructions until fully heated. Do not stir the batter.
Lightly spray or brush the waffle iron with cooking spray or melted butter. Pour batter into the center of the iron, using about 3/4 cup for a standard Belgian waffle maker.
Close the lid and cook for 4-6 minutes, or until steam stops escaping from the sides and the waffle is deep golden brown. Avoid opening the iron too early.
Carefully remove the waffle using a fork or tongs and place on a wire rack. Repeat with remaining batter, keeping finished waffles warm in a 200°F oven if desired.
Serve immediately while crispy, topped with your favorite toppings. The batter will keep refrigerated for up to 24 hours.
