Preheat the oven to 375°F and generously grease a 2 1/2 quart casserole dish or skillet with butter.

Bring a large pot of water to a boil, add 1 Tbsp salt and 1 Tbsp Old Bay Seasoning, then cook the shrimp for 30 seconds until opaque; immediately remove, cool for a few minutes, and drain any accumulated water.

Combine the shrimp with mayonnaise, garlic powder, and black pepper, stirring to coat, then fold in both shredded cheeses and transfer the mixture evenly into the prepared casserole dish.

Melt 1 Tbsp butter, mix it with 1/2 cup panko bread crumbs until moistened, and sprinkle this mixture evenly over the shrimp casserole.

Bake uncovered for 13-15 minutes at 375°F until the center reaches 145°F, then broil for 1-2 minutes to brown the crumbs; allow the casserole to rest for 15 minutes before serving.