Once popovers have cooled completely, they can be stored at room temperature in zipper-lock bag for 2 days. To serve, adjust oven rack to middle position and heat oven to 400 degrees. Heat popovers on rimmed baking sheet until crisp and heated through, 5 to 8 minutes.
Greasing the pans with shortening ensures the best release, but cooking spray may be substituted; do not use butter. To gauge the popovers’ progress without opening the oven door, use the oven light during baking. Bread flour makes for the highest and sturdiest popovers, but an equal amount of all-purpose flour may be substituted.