Preheat oven to 400. Split muffins in half, and arrange on a baking sheet, cut side up. Spread each muffin with 1-2 T. sauce, depending on how saucy you like your pizza. Top with cheese. Top with pepperoni, if desired. Bake at 400 for 12-15 min, or until cheese is melted and beginning to brown. Remove from oven. Let stand 5 min before serving.