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.
4 boneless skinless chicken breasts
Add olive oil, salt, pepper, garlic powder, and Italian seasoning to the bowl and toss to combine/coat the chicken.
1/2 tablespoon olive oil, salt and pepper to taste, 2 teaspoons garlic powder, 2 teaspoons Italian seasoning
Pour half of the marinara into the bottom of the baking dish and spread into a thin layer.
1 jar marinara sauce
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.
1 jar marinara sauce, 4 slices thick cut mozzarella cheese, 30 slices pepperoni, 1/8 cup grated parmesan cheese
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.
1 jar marinara sauce
Top the marinara with the shredded mozzarella, remaining pepperoni, and remaining grated parm. Sprinkle with more Italian seasoning if desired.
30 slices pepperoni, 1 cup shredded mozzarella cheese, 1/8 cup grated parmesan cheese
Bake for 25 minutes or until the internal temperature on the chicken reaches 165F.