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; blend until combined. 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.]

Prepare your muffin pan or popover pan. Cut the remaining 4 tablespoons of butter into 12 equal pieces, and put one piece 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. I recommend putting a baking sheet on the rack below the muffin pan to catch butter drips that might make a mess of the bottom of your oven.

Remove the pan 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 pan to the oven and don't even think about opening the oven door!! The popovers won't rise properly if you let the steam out of the oven. 

After 20 minutes, reduce the heat to 375° F (190° C) and continue 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.