Lobster Mac and Cheese

Name: Lobster Mac and Cheese
Ingredients: Two 1 1/4-pound lobsters; 3 tablespoons unsalted butter; 1 shallot, minced; 2 to 3 tablespoons cognac; 1 cup shrimp stock; 1/2 cup heavy cream; Juice of 1/2 lemon; 1 sprig fresh tarragon, coarsely chopped; 1 quart (4 cups) heavy cream; 1/2 cup finely grated Parmesan; 2 tablespoons Dijon; 2 teaspoons Worcestershire sauce; 1 teaspoon hot sauce, such as Tabasco; 1/4 teaspoon freshly ground black pepper; Kosher salt; 3 to 3 1/2 cups grated Gruyere; 1 pound elbow macaroni