Preheat oven to 350°. In a large skillet, cook beef over medium heat until no longer pink, 5-7 minutes, breaking into crumbles; drain. Stir in pizza sauce, pepperoni and American cheese. Cook and stir until cheese is melted, 4-5 minutes.
Place buns on a baking sheet, cut sides up. Spoon meat mixture onto buns; top with mozzarella cheese. Bake until cheese is melted, about 5 minutes. If desired, serve with additional warmed pizza sauce.
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