Preheat oven to 400º. Spray a large baking dish with nonstick spray.
Butterfly each chicken breast but do not cut all the way through. Place the chicken breasts in a large bowl.
Add olive oil, salt, pepper, garlic powder, and Italian seasoning to the bowl and toss to combine/coat the chicken.
Pour half of the marinara into the bottom of the baking dish and spread into a thin layer.
Take one chicken breast and open to expose the inside of the butterfly cut.
Place slice mozzarella inside chicken breast and then top with 5 slices pepperoni. Add a spoonful of marinara and then top with a sprinkle of grated parmesan. Close the chicken breast (you can secure with toothpicks, but that is optional. If you use toothpicks don’t forget that they’re there!) and lay in baking dish. Repeat with all four chicken breasts.
Top the chicken breasts with the rest of the marinara and use a spoon to make sure the breasts are fully covered. This will make sure the chicken is super moist and doesn’t dry out.
Top the marinara with the shredded mozzarella, remaining pepperoni, and remaining grated parm. Sprinkle with more Italian seasoning if desired.
Bake for 25 minutes or until the internal temperature on the chicken reaches 165F.