Important: The key to making perfect popovers is having the eggs and milk at room temperature or warm before mixing. It is also important to let the batter sit for an hour before baking it. Popovers do not freeze well and pre-made batter has a tendency not to work properly the next day.
Place the milk in a bowl and microwave on high for approximately 1 1/2 minutes, or until just warm to the touch or registers 110 degrees F. on your cooking thermometer.
Spoon the flour into a cup and level it off with a knife (do not shake the cup to settle the flour). In a large bowl, sift the flour, salt, and baking powder together
In a blender or food processor, combine the eggs, milk, melted butter, and Dijon mustard; process approximately 10 seconds until blended.
Add flour mixture to egg mixture in blender. Process approximately 10 to 15 seconds or until just combined. Only mix the batter until small lumps are left in it, to reduce the risk of over mixing the batter. Scrape down the sides of the blender with a rubber spatula, if necessary.
Let batter rest for 60 minutes before pouring into the popover cups. DO NOT refrigerate the batter.
Preheat oven to 450 degrees F. Move the oven rack to a lower (but not necessarily the lowest) slot to keep the tops from overbrowning.
TIP: To prevent the popover tops from browning too quickly, position an oven rack at the very top of the oven, and put a cookie sheet on it to shield the popover tops from direct heat.
Place the empty popover pans in the oven for 5 minutes or until they are hot.
Once oven is the correct temperature and the empty popover pans are hot, quickly remove the popover pan from the oven; lightly spray the popover cups with a nonstick spray (the popover pans only need a thin coating of baking spray to prevent sticking).
Fill the popover cups with the prepared popover batter almost to the top with the batter. If you leave one of the cups of the popover pan empty, fill it half full of water (this will help protect the pan from the high heat). Immediately place the pan back into the oven.
Bake without opening the door for 15 minutes. Do not peek - this is important!
Reduce heat to 375 degrees F. and bake for another 20 to 25 minutes or until deep golden brown on the outside and airy on the inside. Under baking can cause popovers to collapse after they’re removed from the oven. Resist the urge to open the oven door. If you open the oven door, the heat escapes, the oven cools down, the steam inside the popovers condenses, and the popovers collapse.
Remove popovers from the oven, and unmold onto a rack. Popovers should pull away from the pan easily; if not, use a dull-bladed knife to nudge them from the pan.
Pierce the sides with the tip of a sharp knife to let steam escape (this will keep the exterior crisp, the interior moist, and prevent the popovers from collapsing).
Serve immediately with lots of butter.
Recipe yields 6 Popovers.
* Bread flour is a high-protein flour. The high protein help the popovers rise. All-purpose flour may be substituted but the results will not be as spectacular.
** Do not use egg substitutes as the popovers will lose their puff if you do.
Perfect Popovers Recipe: https://whatscookingamerica.net/Bread/Popovers.htm