Preheat the oven to 375°F (190°C). Grease a 2 1/2-quart casserole dish with butter.

Fill a large pot with water, add 1 tablespoon salt and 1 tablespoon Old Bay, and bring to a rolling boil.

Add the shrimp to the boiling water. Cook just until opaque, about 30 seconds — large shrimp cook very fast. Immediately drain and transfer to a mixing bowl.

Add the mayonnaise, garlic powder, and black pepper to the bowl. Stir to coat every shrimp evenly.

Stir in the shredded mozzarella and cheddar until the mixture is uniform.

Spoon the shrimp-cheese mixture into the prepared casserole dish, spreading it into an even layer.

Melt 1 tablespoon butter and combine with the panko until moistened. Sprinkle the breadcrumb mixture over the shrimp.

Bake uncovered for 13–15 minutes, or until the center reaches an internal temperature of 145°F. For a golden crust, broil 1–2 minutes at the end, watching carefully.

Let the casserole rest 15 minutes before serving so the filling firms and is easier to portion.