Preheat the oven to 450 degrees. Coat a cast iron skillet (or baking dish) with cooking spray.
Place the stemmed mushrooms top-side down in the skillet.
Spoon a small dollop of pizza sauce into each mushroom then top with a bit of shredded mozzarella cheese.
Add a few pepperoni slices to the top of each mushroom then sprinkle some crushed red pepper flakes and freshly cracked black pepper, to taste, if desired.
Combine the melted butter with the minced garlic; set aside until needed.
Bake for 12 minutes then pour the melted garlic butter evenly into the bottom of the skillet around the mushrooms.
Return to the oven to continue baking for 3-5 minutes, or until the cheese is melted and a bit golden on top. Remove from the oven.
Sprinkle with fresh basil and Parmesan cheese, if desired. Serve and enjoy!