1) Preheat the oven to 450°F. Position a rack on a lower shelf. The top of the fully risen popovers should be about midway up the oven. What you don't want is for the tops of the popping popovers to be too close to the top of the oven, as they'll burn.
2) Use a standard 12-cup metal muffin tin, one whose cups are close to 2 1/2" wide x 1 1/2" deep. Grease the pan thoroughly, covering the area between the cups as well as the cups themselves. Make sure the oven is up to temperature before you begin to make the popover batter.
3) Use a wire whisk to beat together the eggs, milk, and salt. Whisk till the egg and milk are well combined, with no streaks of yolk showing.
4) Add the flour all at once, and beat with a wire whisk till frothy; there shouldn't be any large lumps in the batter, but smaller lumps are OK. OR, if you're using a stand mixer equipped with the whisk attachment, whisk at high speed for 20 seconds. Stop, scrape the sides of the bowl, and whisk for an additional 20 to 30 seconds at high speed, till frothy.
5) Stir in the melted butter, combining quickly.
6) Pour the batter into the muffin cups, filling them about 2/3 to 3/4 full.
7) Make absolutely certain your oven is at 450°F. Place the pan on a lower shelf of the oven .
8) Bake the popovers for 20 minutes without opening the oven door. Reduce the heat to 350°F (again without opening the door), and bake for an additional 10 to 15 minutes, until they're a deep, golden brown. If the popovers seem to be browning too quickly, position an oven rack at the very top of the oven, and put a cookie sheet on it, to shield the popovers' tops from direct heat.
9) If you plan on serving the popovers immediately, remove them from the oven, and stick the tip of a knife into the top of each, to release steam and help prevent sogginess. Slip them out of the pan, and serve.
10) If you want the popovers to hold their shape longer without deflating and settling quite as much, bake them for an additional 5 minutes (for a total of 40 minutes) IF you can do so without them becoming too dark. This will make them a bit sturdier, and able to hold their "popped" shape a bit longer.