Bring a large pot of water to a boil over medium-high heat. Salt generously and cook the pasta until al dente, 10 to 12 minutes depending on the shape. Drain the pasta and set aside.

In the same pot, melt the butter. Add the flour and cook until lightly toasted.

Meanwhile, dissolve the mustard powder in the warm water. Add the milk and mustard to the pot and whisk until smooth. Cook over medium-low heat until the sauce thickens slightly.

Whisk in eggs, 1/2 teaspoon salt, and the cayenne and stir constantly with a flat-edged wooden spoon, trailing the bottom of the pan to make sure it doesn’t curdle, until the sauce thickens, about 5 minutes. Stir in the Cheddar and Gruyère cheeses over low heat. Remove from the heat as soon as about half the cheese is melted. Continue stirring off the heat until all the cheese melts. Add the cooked pasta and stir to coat.

Preheat the broiler/grill to high.

Transfer the macaroni to a 1-quart ovenproof baking dish and lay the Muenster cheese over the top. Broil on the top rack of the oven until golden and bubbly, about 2 minutes. Top with freshly ground pepper. Serve immediately.