Bring 6 cups milk and 1 ½ teaspoons salt to a simmer over medium heat in a large pot, such as the Cravings braiser. Add the shells, return to a simmer, reduce the heat to medium-low and cook, stirring occasionally, until the pasta is al dente, 9–10 minutes.
Reduce the heat to the lowest possible and add the cheese, butter, mustard, cayenne, and remaining 1 ½ teaspoons salt and stir until the cheese is fully melted and the sauce thickens slightly, 2–4 minutes. Use up to ½ cup milk to thin the sauce, if needed.
