Preheat oven to 400°F, and grease popover pan.
Add milk, eggs, salt, melted butter, and flour to FourSide or WildSide jar. Secure lid and select "Batter."
Preheat popover pan in oven for 2 minutes. Remove pan, and place 1 piece of unmelted butter in each cup; place pan back in oven until butter is melted.
Remove pan from oven, and fill each cup ½ full with batter. Bake for 20 minutes. Reduce temperature to 300°F, and continue baking for 12–15 minutes. Do not open oven during baking.