Step 1

Preheat oven to 350°. Cook macaroni in a 6-qt. stockpot according to package directions for al dente. Drain; return to pot.

Step 2

In a large saucepan, melt 6 tablespoons butter over medium heat. Stir in flour until smooth; whisk in warmed milk. Bring to a boil, stirring constantly; cook and stir until thickened, 2-3 minutes.

Step 3

Remove from heat; stir in cheeses, salt, pepper and ground mustard. Add to macaroni, stirring to coat. Stir in shrimp.

Step 4

Transfer to a greased 13x9-in. baking dish. In a small bowl, melt remaining 2 tablespoons butter. Stir in bread crumbs and Parmesan cheese; sprinkle over casserole. Bake, uncovered, until shrimp turn pink and casserole is bubbly, 30-40 minutes.