Preheat oven to 425 degrees F (220 degrees C).
Spray a popover pan with nonstick cooking spray. Place pan on center rack of oven and preheat for 2 minutes.
Blend flour, salt, eggs, milk, and melted butter until it looks like heavy cream, about 1 to 2 minutes.
Cut chilled butter into 6 even pieces. Place 1 piece of butter in each cup and place pan back in oven until butter is bubbly (about 1 minute).
Fill each cup half full with batter and bake 20 minutes. Reduce heat to 325 degrees F (165 degrees C) and bake for another 15 to 20 minutes.
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