Preheat the oven to 450 F (230 C). In a small bowl, stir together the flour, salt, and baking powder. In a blender, combine the eggs, milk, and 3 tbsp melted butter and blend until smooth. Add the flour mixture and blend until smooth, scraping down the sides with a rubber scraper as needed.
[Alternatively, whisk together the wet ingredients in a large bowl, and then whisk in the dry ingredients until a smooth batter forms. Pour into a 4-cup liquid measuring cup with a spout.]
Place a nonstick 12-cup muffin pan or popover pan on a rimmed baking sheet. Cut the 4 tablespoons of butter into 12 equal pieces, and place 1 pice in the bottom of each cup. Place the pan in the oven until the butter melts and starts to sizzle and pop, about 5 minutes.
Remove the baking sheet from the oven and quickly pour the batter into the muffin/popover cups, filling them three-fourths full (a bit more for the muffin pans). Return the baking sheet to the oven and don't even think about opening the over door, or the popovers won't rise.
After 20 minutes, reduce the heat to 375 F (190 C) and continue to baking until crisp and golden 15-20 minutes longer. Turn the popovers out and serve right away. Fresh jam is highly recommended.
*I'm serious about not opening the oven, so you may need to plan ahead if you are making these alongside a meal that also needs the oven. Hopefully you have a double oven in that case. Or use a neighbor's. Or make something that doesn't need the oven.
Source: Williams-Sonoma's Family Meals Cookbook by Maria Helm Sinskey