Preheat the oven to 450°F. Set a popover or muffin pan inside to heat.
Whisk eggs and milk until foamy and well combined.
Add flour and salt. Whisk just until smooth, then stream in melted butter.
Let the batter rest 10 minutes while the pan gets hot.
Carefully remove the hot pan. Brush cups lightly with a bit of melted butter.
Fill each cup about halfway with batter.
Bake 20 minutes at 450°F without opening the oven door.
Reduce heat to 350°F and bake 12–15 minutes more until deeply golden and puffed.
Pierce each popover with a knife to vent steam. Serve immediately.