Preheat oven to 375°F (190°C).
Cook spaghetti in boiling salted water according to package instructions until al dente, then drain.
In a small bowl, whisk together melted butter, minced garlic, and chopped parsley.
Slice the top off each bread loaf and hollow out the center, leaving a 0.5-inch thick wall.
Brush the inside and rim of each bread bowl with the garlic butter mixture.
Toss the drained spaghetti with marinara sauce until well combined.
Fill each bread bowl with the spaghetti and sauce mixture.
Top each bowl with mozzarella and Parmesan cheese.
Bake on a parchment-lined baking sheet for 12 to 15 minutes until cheese is melted and bread is golden brown.