Preheat and prep: Preheat oven to 375°F (190°C). Grease a 2 1/2 quart casserole dish with butter.
Cook shrimp: Bring a large pot of salted water to a boil. Add Old Bay seasoning. Add shrimp and cook until opaque, about 30 seconds. Drain and transfer to a bowl.
Combine ingredients: Add mayonnaise, garlic powder, and pepper to the shrimp. Stir to coat. Add cheeses and stir again. Transfer to the prepared casserole dish.
Make topping: Melt butter and stir in bread crumbs. Sprinkle over the shrimp mixture.
Bake: Bake uncovered for 13-15 minutes, or until the center reaches 145°F. Broil for 1-2 minutes to brown the crumbs. Rest for 15 minutes before serving.