Begin by bringing a large pot of salted water to a boil and adding a tablespoon of Old Bay seasoning. Carefully drop in the shrimp, cooking for about 2-3 minutes until they turn pink and opaque. Drain and set aside to cool slightly.

In a large mixing bowl, gently fold the cooked shrimp with mayonnaise, garlic powder, black pepper, and shredded mozzarella and cheddar cheese until well combined.

Grease a casserole dish with butter, then transfer the shrimp mixture into the dish, spreading it evenly.

In a small bowl, melt a few tablespoons of butter and add panko bread crumbs, stirring until well-coated.

Preheat the oven to 350°F (175°C) and bake the casserole for 20-25 minutes until the top is golden brown and the filling is bubbling.