Preheat the oven to 375 degrees F.
Bring a large pot of well salted water to a boil. Add the pasta and cook according to the directions on the package, 6 to 8 minutes. Drain well.
Gently heat the milk in a small saucepan. In a separate large pot, melt 6 tablespoons of butter and add the flour. Cook, stirring with a whisk, over low heat for ~ 2 minutes. While whisking, add the hot milk and cook, stirring, for a minute or two, until thickened and smooth. Add the cognac (if using) and simmer for 1 minute. Turn off the heat, add the Gruyere, White Cheddar, 1 tablespoon salt, the pepper, and nutmeg. Add the cooked pasta and lobster and stir well. Place the mixture in 6 to 8 individual gratin dishes.
Melt the remaining 2 tablespoons of butter, combine them with the fresh bread crumbs, and sprinkle on the top. Bake for 30 to 35 minutes, or until the sauce is bubbly and the macaroni is browned on the top.