Preheat your oven to 375°F (190°C) and grease a 9x13-inch casserole dish with butter.

Bring salted water to a boil, add Old Bay seasoning, then cook the shrimp for about 30 seconds until just opaque. Drain and transfer to a mixing bowl.

Add mayonnaise, garlic powder, and black pepper to the shrimp, mixing gently until coated.

Fold in mozzarella and cheddar cheese until well integrated.

Transfer shrimp and cheese mixture into the prepared casserole dish and spread evenly.

Melt butter in a small saucepan, then mix in panko breadcrumbs until coated. Sprinkle over the casserole.

Bake uncovered for 13-15 minutes until the center reaches 145°F and edges bubble.

Switch oven to broil and broil for 1-2 minutes until the topping is golden, watching carefully.

Let the casserole rest for about 15 minutes before serving.