Preheat the oven to 450°F. Make sure one of the racks in your oven is positioned in the lower third of the oven and the top rack is as high as it can be so that your popovers have plenty of room to ‘pop’.
Place 1 teaspoon of butter in each popover cup of a 6-cup standard popover pan. Place the popover pan on a baking sheet and place it in the oven.
In a blender, combine the eggs and warm milk. Blend until frothy.
Add the flour, salt, and remaining 3 tablespoons of melted butter. Blend for another 30-60 seconds until the batter is smooth and no clumps of flour remain.
Remove the hot popover pan from the oven and divide the batter evenly between the 6 popover cups. Smooth the tops with a rubber spatula. The batter should fill each cup about ¾ of the way.
Bake for 20 minutes. Without opening the oven door, reduce the temperature to 350°F and bake the popovers for 10-15 more minutes. The popovers should be nicely browned.
Remove the popovers from the oven and serve immediately!
