To get the full dramatic effect of popovers, serve them as soon as you take them from the oven. The big “puffs” will start to deflate almost immediately. You can prepare the popover batter (step 2) up to 24 hours before baking them. Store the batter in a covered container in the refrigerator. If you use the batter directly from the refrigerator, the baking time may be increased by 2 or 3 minutes.
Source: The CIA’s New Book of Soups cookbook.