Boil a large pot of water on medium-high heat, generously salting it. Cook pasta until al dente (10-12 minutes). Drain and set aside
In the same pot, melt butter. Add flour, toasting it lightly. Dissolve mustard powder in warm water. Add milk and mustard to the pot, whisking until smooth. Cook until the sauce slightly thickens
Whisk in eggs, 1/2 teaspoon salt, and cayenne pepper. Stir constantly with a flat-edged wooden spoon, preventing curdling, until the sauce thickens (about 5 minutes)
Stir in Cheddar and Gruyere cheeses over low heat. Remove from heat when about half the cheese is melted. Continue stirring off the heat until all the cheese is melted. Add the cooked pasta and stir to coat
Preheat the broiler to high. Transfer the macaroni to an ovenproof baking dish and layer Muenster cheese on top. Broil on the top oven rack until golden and bubbly (1-2 minutes max)
Finish with freshly ground pepper. Serve promptly