1. Combine first 4 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, salt, and pepper, stirring with a whisk. Gradually add flour mixture to milk mixture, stirring well with a whisk.
4. Coat 6 popover cups with cooking spray, and brush melted butter evenly among cups to coat. Place the 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 the popovers immediately.