Preheat the oven to 450°. Generously butter a popover pan or 2 mini-muffin pans. In a medium bowl, whisk the milk with the flour and salt; there will be some lumps. Add the eggs and butter and whisk just until blended.
Heat the prepared pan or pans in the oven until the butter is sizzling, about 1 minute. Carefully spoon the popover batter into the pan or pans: Fill the popover pan two-thirds full for giant popovers or fill the mini-muffin pans three-quarters full for mini popovers. Bake the popovers for 15 minutes without opening the oven, then lower the temperature to 350° and bake the minis 15 minutes longer, or the giants 20 minutes longer. Transfer the popovers to a baking sheet, poke a small hole in the tops with a tooth-pick and then dry them out in the oven for another 10 minutes. Serve the popovers warm.