Preheat oven to 375°.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and salt, stirring with a whisk. Combine milk and eggs in a medium bowl, stirring with a whisk until blended; let stand 30 minutes. Gradually add flour mixture, stirring well with a whisk. Stir in butter.
Coat 9 popover cups with cooking spray; brush oil evenly among cups to coat. Place popover cups in a 375° oven for 5 minutes. Divide the batter evenly among prepared popover cups. Bake at 375° for 40 minutes or until golden. Serve immediately.