The ingredients should be at room temperature. If you forgot to set them out earlier, you can put the milk in a jar and then put the jar and the eggs in warm water to warm them up quickly.
Preheat oven to 425.
Put the popover pan in the oven to heat it.
Microwave the butter to melt it. Be careful not to let it burn; it doesn't take long.
Beat together the eggs, milk, 1 tablespoon of the melted butter, sugar, and salt.
Beat in the flour a little at a time.
Remove the popover pan from the oven. Spoon the rest of the butter into each cup of the popover pan, dividing the butter equally.
Divide the batter evenly among the cups. Work quickly so pan doesn't cool off. Be sure not to overfill the cups: big cups, 1/3 full at most, and thin cups, 1/2 full. A full recipe makes about 18 popovers if you are careful not to overfill the cups. It is much better to have more fluffy ones than overfill and have fewer flat ones.
Once you've filled the cups with batter, immediately put the pan back into the oven.
Bake for 15 or 20 minutes until done. They should be nicely browned.