Heat oven to 450° F.
With fingers or a pastry brush, use 1 teaspoon of the butter to coat 6 cups in a popover or muffin tin.
Whisk the eggs in a small bowl. Whisk in the milk and the remaining butter.
In a large bowl, combine the flour and salt. Slowly whisk the eggs into the flour, just until combined. Divide the batter evenly among the cups, filling them about halfway. Bake for 15 minutes (don't open oven door). Lower temperature to 350° F and bake until puffed and golden, 15 to 20 minutes more. Check one popover by breaking it open. If it is slightly underdone (moist, not airy), turn off the oven and leave the popovers inside for 5 to 10 minutes more.