First, preheat your oven to 400 degrees Fahrenheit.
Cut chicken breasts in half lengthwise to create 6 chicken cutlets. Sprinkle both sides evenly with the garlic salt and black pepper.
In a large, oven-proof skillet, melt the butter over medium high heat. Cook the chicken on both sides, about 2 – 3 minutes per side. Remove from heat. The chicken does not have to be cooked all the way through at this point. It will finish baking in the oven.
Spoon the marinara sauce over the chicken.
Next, sprinkle the cheeses and Italian seasoning evenly over the top.
Place pepperoni slices on top.
Transfer the skillet to the oven and bake uncovered for 20 minutes.
Remove from the oven, let rest for 5 minutes, and serve.