With your mixer beat eggs, milk, flour, and salt until smooth. Refrigerate batter until ready to use or up to 2 days.
Preheat oven to 425 degrees. Grease 2 1/2 inch muffin cups. Place 1/2 teaspoon butter or margarine in each cup and heat in oven until butter sizzles. Pour batter into cups filling each 1/2 to 2/3 full. Bake 25 to 30 minutes or until popovers are puffed and brown. Do not peak! Serve immediately with butter.