Preheat your broiler
Bake or microwave your potato until fork tender. Let it cool.
Slice the potato in half, lengthwise. Spoon out the potato being careful to not go into the skin.
Add the garlic salt to the melted butter and give a good stir.
Brush the interior, and exterior of the potato skins with the salted butter.
Place onto a baking sheet and place under the broiler. Keep an eye on the skins as you want them to get lightly crispy and a deep golden brown. Once that is achieved, remove them from the broiler.
Place about a 1/4 cup of the marinara sauce into each potato skin and use a spoon to smooth the sauce so it cover the inside. Add a 1/4 cup of mozzarella cheese into each potato skin, then divide the pepperoni into each potato skin, tucking them into and onto the cheese.
Place back under the broiler and cook until the cheese is fully melted, the pepperoni is slightly crisp, and when those babies are looking awesome to your liking. This is the time to get excited!