Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Melt the butter in a skillet over medium heat. Add the shrimp and cook just until pink, about 2–3 minutes. Season lightly with salt, pepper, and paprika. Remove from heat right away to keep the shrimp tender.
In a large bowl, stir together the cooked rice, sour cream, mayonnaise, onion, garlic, cheddar, mozzarella, garlic powder, a little salt and pepper, and cayenne if using. Mix until smooth and creamy.
Gently fold the warm shrimp into the rice mixture. Stir just until everything is evenly coated.
Spoon the mixture into the prepared baking dish and smooth the top. Add a little extra cheese or a light sprinkle of crushed crackers/breadcrumbs if you like a golden topping. Bake for 25–30 minutes, until bubbling and lightly golden around the edges.
Let it rest for 5 minutes. Sprinkle with parsley and serve warm.
