Preheat the oven to 425ºF (215°C) and position the rack on the lowest third.
Butter nine ¾ cup glass custard cups or ten ½ cup muffin cups with 2 tablespoons unsalted butter, soft or melted. If you're using individual custard cups, place them on a jelly roll or oven tray, leaving space between them. If you're using muffin pans, you'll need to use two 12-muffin tins because you will fill them alternating, leaving one empty in between to give space for the air to circulate between the popovers while baking.
Place 1 cup all-purpose flour, 1 cup whole milk, ½ teaspoon salt and 3 eggs in a blender jar and whirl until smooth. You can also use a food processor, or bowl and an immersion mixer. Strain the batter if it has lumps. Let the batter rest for 15 minutes. At this point, you can cover the jar or bowl and refrigerate the batter for up to 1 day.
While the batter is resting, place the buttered cups for 10 minutes in the hot oven. They will smoke as the butter darkens. The hot pans will make the popovers rise beautifully. Have the batter ready in a pitcher, jar or large measuring cup to fill them immediately a you take them out.
If using custard or popover cups, fill them ⅓ of their capacity, dividing any extra batter among them. If using muffin pans, use ¼ cup of batter for each cup, alternating between them so that every popover has puffing space around.
Bake, without opening the door, for 25 minutes or until the popovers are puffed, nicely browned, and crisp on the exterior.
Turn the temperature down to 350ºF (180°C) and bake for another 5 to 10 minutes, to help dry out the interior, which will always be a little doughy in the center. Some people love this part, others pull it out.
Serve immediately. Popovers are at their puffiest right out of the oven.
You can keep them at room temperature for a few minutes, or wrap them airtight and freeze them for up to a month, and reheat them in a 350ºF (180°C) oven for 10 to 15 minutes. They'll taste good, but never as good as freshly baked.