Combine chopped shrimp, cream cheese, mozzarella, parsley, garlic, salt, pepper, and optional red pepper flakes until creamy.
Preheat oven to 375°F. Slice loaf in half lengthwise and scoop out the soft center, leaving a sturdy shell.
Evenly spread the shrimp mixture into both halves, pressing gently.
Brush outsides with melted butter. Wrap each half tightly in foil.
Bake wrapped loaves for 15 minutes. Unwrap and bake 5 more minutes to crisp.
Cool slightly, slice into 2-inch pieces, and serve warm.