In a medium-sized bowl, whisk together the flour, sugar, and salt until combined.
In a separate bowl, whisk together the milk, egg melted butter, and vanilla extract until smooth.
Add the wet ingredients to the dry ingredients and whisk until just combined - do not overmix. Let the batter rest for 15 minutes.
Heat your waffle iron and lightly coat it with cooking spray or melted butter. Pour 1/4 cup of batter into the center of the waffle iron and spread it out evenly. Close the iron and cook until golden brown, about 2-3 minutes.
Carefully remove the waffle from the iron and transfer it onto a plate. Repeat with the remaining batter. Serve immediately or keep warm in an oven set to 200°F (93°C).