Preheat oven to 350 degrees F (175 degrees C). Put 12-cup muffin tin in oven to preheat as well.

Beat eggs and milk together in a bowl with an electric hand mixer. Add flour and salt; continue beating into smooth batter.

Put about 3/4 teaspoon butter into each muffin tin and carefully spread butter up sides of each cup. Quickly divide batter among prepared cups.

Bake until popovers are risen well and browned, 30 to 35 minutes. Do not open the oven during baking.