Boil the pasta shells in salted water for 1 to 2 minutes less than package directions. Drain but do not rinse.

Melt 2 tbsp butter in a large saucepan over medium heat, add 2 tbsp flour, and whisk for 1 minute until pale and nutty.

Slowly whisk in 1 cup whole milk and 1 cup heavy cream, then simmer over medium heat for 3 to 4 minutes until the sauce coats a spoon.

Reduce heat to low. Stir in 1 cup cheddar, ½ cup Gruyère, and ½ cup fontina in batches until melted, then stir in ¼ cup Parmesan, ½ tsp garlic powder, ½ tsp mustard powder, ½ tsp salt, and ¼ tsp white pepper.

Fold in the drained pasta, taste for salt, and transfer to a buttered oven-safe skillet or baking dish.

Toss ¼ cup panko with 1 tbsp melted butter, spread over the pasta, dust with ¼ tsp smoked paprika, and broil on high 6 inches from the element for 2 to 3 minutes until deep golden brown. Serve immediately.