If milk and eggs are straight out of the fridge, heat milk in a small saucepan just till small bubbles form around the edge, and submerge whole eggs in warm water for 10 minutes. Heat oven to 450 degrees with a nonstick popover pan on rack in lowest position.
Mix salt and flour and set aside. Lightly whisk eggs, then add milk in a slow stream and whisk together till very frothy, about one minute.
Add flour and salt to egg mixture, whisking by hand just till batter is the consistency of heavy cream with some small lumps and air bubbles remaining.
Remove popover pan from oven then coat sides and tops of cups with cooking spray. (If using a standard muffin tin, only coat and fill the outer cups, and reduce baking time by 5 minutes).
Fill popover cups about 3-quarters full with batter. Bake for 20 minutes, then reduce oven temperature to 350 degrees and continue baking until golden brown and dry to the touch, for about 20 minutes more. Don’t open the oven door to peek while the popovers are baking!
Popovers are butter served warm, turn them out of the pan right away, as they tend to lose their crunch if they stay in the hot pan. If the rolls will be sitting, using a knife poke a small opening in the top of each to let the steam escape and preserve their crispiness.
