Preheat oven to 425 degrees F. Generously grease muffin tin or popover tin with melted butter. Set aside.
Using an immersion blender (or standard blender), mix together milk and eggs until there are no longer any streaks of whites or yolks. Add the flour and salt, and pulse until combined. Add the melted butter and pulse until batter is thoroughly incorporated. Alternatively, whisk the batter by hand.
Fill the greased muffin tin or popover tin 2/3- 3/4 full of batter. Place the tin in the center or bottom third of the oven. Bake for 20 minutes undisturbed. Do not open to peek at the popovers during this time. The batter needs all the heat from the oven to rise. After 20 minutes, the baking time will vary depending on the tin used. Bake for another 13 to 15 minutes if using a standard muffin tin. Bake for another 18 to 20 minutes if using a popover tin.
The finished popovers should be golden brown on the outside and feel light in weight. They should be crisp on the outside and slightly soft and chewy on the inside. Serve popovers warm. Popovers are best served with butter or jam, or with gravy or sauce from pot roast or roast beef.
Notes
To make a cheese variation: add 1/2 cup shredded cheese to the batter.
Alternatively, you can also add herbs and spices to taste.