In a saucepan, melt butter and 1/2 teaspoon of garlic powder. Cut each roll into thirds; roll each piece into a ball. Dip dough balls in butter mixture; place along outer edge of a 10-inch cast-iron skillet. Carefully place remaining dough balls on top of bottom layer, leaving some space between them and not placing in the center. Cover and rise in a warm place until almost doubled, about 30 minutes. (Time will vary depending on how warm your kitchen is.)

Preheat oven to 400 degrees F. Bake until dough balls are set and just begin to brown, 15-18 minutes.

Meanwhile, combine cream cheese, 1 cup mozzarella, mayonnaise, diced onion, and the remaining garlic powder; spoon into center of skillet. Layer with 1/4 cup mozzarella and pizza sauce. Top with remaining mozzarella and pepperoni. Brush rolls with some of remaining butter mixture; sprinkle with Parmesan.

Bake until dip is heated and the rolls are golden brown, about 10 to 15 minutes. Cover edges with foil if they begin to over-brown.