Preheat oven to 425 degrees F (220 degrees C). Place pumpkin halves on a baking sheet, cut-side up. Spread butter on the inside of each half.
Bake in the preheated oven until flesh is tender, about 50 minutes.
Combine beef, pork, spinach, cauliflower rice, onion, cinnamon, salt, pepper, garlic powder, and cayenne pepper; fold in 1/2 of the Parmesan cheese. Fill each pumpkin half with mixture; top each with remaining Parmesan cheese.
Bake in the oven until filling is cooked through, about 10 minutes more.