Boil the cavatappi in water with 1 teaspoon salt for 3 minutes less than the package directs and drain.
Butter a 6-quart crockpot and add the pasta, evaporated milk, whole milk, garlic, pepper and remaining salt.
Cook on low for 45 minutes, then stir in the cheddar and 1 1/2 cups Parmesan and cook for 45 minutes more.
Transfer the hot macaroni to a shallow broiler-safe 9-by-13-inch dish; do not broil the crockpot insert.
Mix the panko, melted butter and remaining Parmesan, scatter it evenly over the macaroni, and broil 5 inches from the heat for 2 to 3 minutes until patchy golden.