Preparation Steps

• Position an oven rack 6 inches from the broiler. Bring a large pot of well-salted water to a boil. In a small saucepan over low heat, gently warm the milk and whisk in the Dijon until smooth; keep warm.
• Grate the cheddar and Parmesan. From the grated cheeses, set aside about 1/2 cup cheddar and 2 tablespoons Parmesan for a bubbly broiled finish.
• Add pasta to the boiling water and cook until 2 to 3 minutes shy of al dente. Reserve 1 cup of pasta cooking water, then drain (do not rinse).
• While the pasta cooks, melt the butter in a wide, heavy pot over medium-low heat. Sprinkle in the flour and whisk continuously, cooking 3 to 4 minutes until the roux turns pale blond and smells nutty.
• Whisk in the garlic powder. Raise heat to medium and slowly stream in the warm milk mixture in 3 additions, whisking until smooth after each. Bring to a gentle simmer and cook, stirring often, until the sauce thickens enough to coat a spoon, about 5 minutes. Season with the salt and pepper.
• Take the pot off the heat and stir in the American cheese until melted and silky. Return the pot to low heat and add the remaining cheddar and most of the Parmesan in two additions, stirring until glossy and smooth.
• Fold the drained pasta into the sauce over low heat. If the mixture seems tight, loosen gradually with a few tablespoons of reserved pasta water until creamy and flowing.
• Gently fold in the lobster and half of the chives; warm for 1 minute without boiling to keep the lobster tender.
• Transfer to a broiler-safe dish, scatter the reserved cheddar and Parmesan on top, and broil on high until the surface is bubbling and golden in spots, 2 to 3 minutes. Let rest 5 minutes, garnish with remaining chives, and serve.