1. Combine first 3 ingredients in a medium bowl, stirring with a whisk until blended. Let stand 30 minutes.
2. Preheat oven to 375°.
3. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours and salt, stirring with a whisk. Gradually add flour mixture to milk mixture, stirring well with a whisk.
4. Coat 6 popover cups with cooking spray; brush butter evenly among cups to coat. Place popover cups in a 375° oven for 5 minutes. Divide batter evenly among prepared popover cups. Sprinkle evenly with cheese. Bake at 375° for 40 minutes or until golden. Serve immediately.