1. Preheat oven to 400 degrees F, placing a 12-cup muffin pan on a baking sheet in the oven to heat with the oven.
2. In a medium bowl, whisk together pumpkin puree, eggs, egg whites, milk, and oil until smooth.
3. Combine flour, salt, pie spice, and cayenne in a large bowl. Whisk the pumpkin mixture into the dry ingredients until smooth.
4. Remove the muffin pan from the oven and coat it with cooking spray. Divide the batter among the prepared cups. Bake the popovers until they are puffed and browned (25-30 minutes).
5. Remove the popovers from the oven and reduce the oven temperature to 350 degrees F. With a small knife, cut small slits into the sides of the popovers, 3 or 4 per popover. Bake until fully cooked (7-10 minutes).