In a small bowl, mix together the butter and garlic powder until well combined.
Spread the garlic butter evenly on the outside of each slice of bread, ensuring to cover the edges.
Sprinkle Parmesan cheese on the buttered side of the bread slices and press it gently into the butter.
Assemble each sandwich in the following order: bread slice (butter side out), shredded mozzarella, warmed spaghetti, provolone cheese slices, and another bread slice.
Heat a well-seasoned cast iron skillet or heavy non-stick skillet over medium-high heat.
Place the assembled sandwiches in the skillet and cook for about 3 minutes on each side, or until golden brown and the cheese is melted.
If possible, cover the skillet with a lid or a sheet pan to help melt the cheese faster.
Once both sides are golden brown and the cheese is gooey, remove the sandwiches from the skillet and serve immediately.