Arrange an oven rack on the middle rung and slide two popover pans (six 6-oz. cups per pan) or one 12-cup nonstick muffin tin (with 3-1/2-oz. cups) onto the rack. Heat the oven to 450°F. In a blender, combine the eggs, milk or half-and-half, melted butter, and mustard. Process until blended, about 10 seconds. Add the flour, salt, and cayenne and blend until just combined, another 10 to 15 seconds. Scrape down the sides, if necessary, and set aside the blended mixture while the oven continues to heat.
Once the oven is hot, moving quickly, remove the pans from the oven, set them on a cooling rack or folded dishtowel, and lightly grease them with the pan spray. Pour the batter evenly into the heated tins (fill each cup one-half to two-thirds of the way full) and immediately slide the pans back into the oven. If you're using popover pans, there will be one empty cup in each pan. If you're using a muffin tin, there should be a little leftover batter. Pour it into a greased 6-oz. custard cup and put it in the oven with the muffin tin (the batter puffs nicely in a custard cup, but the popover may stick to the sides).
Bake without opening the door until the popovers are puffed and well browned, about 25 min. Resist the urge to open the oven door, especially during the first 20 min. of baking -- if you do, you're likely to end up with pancakes instead of popovers. If you like drier popovers, turn the oven off after 25 min. and let the popovers sit there for a few more minutes. Pierce each popover with the tip of a knife to let steam escape; this will keep the exterior crisp and the interior moist, and prevent the popovers from collapsing. Gently tip the pan onto a rack and let the popovers tumble out. Serve immediately with lots of butter. Leftovers can be reheated in a 300°F oven until they're warm. They won't retain their super puff or crisp exterior, but they'll still be delicious.