Position a rack on the lowest rung of the oven and preheat the oven to 425 degrees.
Butter or spray popover or muffin pans.
Pour all the ingredients into the container of a blender and whirl until smooth.
(This can also be done in a food processor or in a bowl, using a rotary or hand-held beater.) Strain the batter if it is at all lumpy.
For the muffin cups, use 1/4 cup of batter for each cup, filling alternate cups so that every popover has puffing space.
Bake, without opening the oven door, for 25 minutes, until the popovers are puffed, nicely browned, and crisp on the exterior.
Turn the temperature down to 350 degrees and bake for another 15 to 20 minutes to help dry out the interior, which, no matter what you do, will always be a little doughy in the center.
(Some people love this part, others pull it out.) Serve immediately.
Storing: Popovers are at their puffiest right out of the oven.
You can hold them at room temperature for a few minutes, or wrap them airtight, freeze them for up to a month, and reheat them in a 350 degree oven for 10 to 15 minutes, and they'll taste good- but never as good as just baked.