1. Preheat oven to 400°.
2. Weigh or lightly spoon flours and cornstarch into dry measuring cups; level with a knife. Combine flours, cornstarch, baking powder, and next 4 ingredients (through salt) in a medium bowl, stirring with a whisk; cut in butter with a pastry blender or 2 knives until the mixture resembles coarse meal. Stir in pepperoni and cheeses.
3. Combine milk and egg in a small bowl; stir with a whisk until combined. Add to flour mixture; stir just until moist.
4. Turn dough out onto a surface lightly dusted with white rice flour. Pat dough into a 6 1/2-inch circle on a baking sheet lined with parchment paper; cut dough into 10 wedges, cutting into but not through dough. Lightly coat top of dough with cooking spray.
5. Bake at 400° for 18 minutes or until edges are golden and center is firm. Remove scones from baking sheet to a wire rack. Serve warm.