Preheat oven to 400 degrees F. Grate cauliflower on the small side of box grater to form fine crumbs. Transfer to a large bowl.
Add egg, 1/3 mozzarella, Parmesan, 2 tablespoons basil, and garlic powder and season with salt and pepper. Form into small patties (they will be wet) and place on a greased baking sheet. Bake until golden, 20 minutes.
Top each cauli patty with a thin layer of marinara, remaining mozz, and mini pepperoni and bake until cheese melts and pepperoni crisps, 5 to 7 minutes more.