1. Preheat the oven to 375 degrees.
2. In a small saucepan, combine the water, butter, and salt. Heat until the butter is melted and mixture boiling. Stir in the flour all at once, stirring vigorously with a wooden spoon.
3. Continue stirring over medium heat until mixture leaves the sides of the pan clean.
4. Remove from the heat and beat in the eggs, one at a time, very well. Stir in all but three tablespoons of the cheese, the mustard, and the cayenne. Drop by teaspoons onto an oiled baking sheet, sprinkle with remaining cheese and bake for 35 to 40 minutes, or until well puffed and golden.
5. Turn off the oven heat and let puffs remain in oven for 3 minutes longer.