Preheat the Oven: Set to 375°F (190°C).
Prepare the Lobster: Cook one lobster, remove the meat, and chop into bite-size pieces. Reserve any juices. Chill until ready to use.
Cook the Pasta: Boil macaroni until almost al dente. Drain, transfer to a bowl, and toss with 2–3 tbsp of butter.
Prepare the Baking Dish: Butter a 9×13-inch baking dish or casserole pan.
Make the Cheese Sauce (Roux): In a saucepan, melt 3 tbsp of butter. Stir in flour to form a thick paste.Gradually whisk in 3 cups of milk, stirring constantly.Bring to a low simmer, then add cheddar cheese and lobster meat with juices.Stir until the cheese melts and the mixture is creamy.Add mustard powder, salt, and pepper (go easy on the pepper).
Combine Pasta and Sauce: Pour the cheese sauce over the macaroni and gently fold to combine.
Add the Topping: Melt 2 tbsp of butter and toss with the panko crumbs. Sprinkle over the macaroni.
Bake: Bake for 30 minutes until bubbly and golden on top.
Finish with Parmesan: Remove from the oven and sprinkle Parmesan across the top. It will melt into the panko and add extra flavor.