Preheat oven to 350°F. Generously butter a 9x13-inch casserole dish and set aside.
Fill a large pot with water. Add 1 tablespoon salt and 1 tablespoon Old Bay seasoning. Bring to a rolling boil over high heat.
Add the peeled and deveined shrimp to the boiling water. Cook for 2-3 minutes, stirring gently, until shrimp turn pink and opaque throughout. Drain in a colander and let cool slightly.
In a large mixing bowl, combine mayonnaise, garlic powder, and black pepper. Stir until seasonings are evenly distributed.
Add the cooled shrimp, shredded mozzarella, and shredded cheddar to the bowl. Fold gently but thoroughly until everything is evenly coated and combined.
Spread the shrimp mixture evenly in the prepared casserole dish, smoothing the top with a spatula.
In a small bowl, melt 1 tablespoon butter. Add the panko breadcrumbs and toss until evenly coated.
Sprinkle the buttered panko mixture evenly over the shrimp, covering the entire surface.
Bake for 15 minutes until the topping is golden brown and the filling bubbles around the edges.
Optional: For extra-crispy topping, switch oven to broil for the last 1-2 minutes, watching carefully to prevent burning.
Let rest for 5 minutes before serving. Enjoy hot!
