Generously grease aluminum popover pans or muffin tins with cooking spray. Place in the oven, then preheat the oven to 425F with the pans/tins inside the oven to heat up.
Whisk together the eggs, milk, melted butter. Gradually add in the salt, sugar and four. Whisk until smooth. The batter should be thin like pancake batter.
Fill the popover pans halfway and bake for 20. Then reduce oven temperature to 350F for 15 additional minutes. DO NOT OPEN THE OVEN WHILE THIS IS BAKING. Or else they will deflate.