You may need to make these in batches. If not serving immediately, poke a small hole in each to release steam. The popovers hold up surprisingly well; reheat just before serving.
Preheat oven to 425 degrees. Whisk together milk, flour, and salt. Whisk in eggs. (Mixture will be lumpy.)
Heat two 6-cup popover pans in oven for 5 minutes, then quickly brush cups generously with butter. Fill each cup a little more than halfway with batter. Bake for 20 minutes.
Reduce oven temperature to 350 degrees. Bake until golden brown, about 25 minutes. Let stand for 5 minutes. Turn out popovers, and serve warm.