In A Large Saucepan, Bring The Water, Butter, Salt And Cayenne To A Boil. Add Flour All At Once And Stir Until A Smooth Ball Forms. Remove From The Heat; Let Stand For 5 Minutes. Add Eggs, One At A Time, Beating Well After Each Addition. Continue Beating Until Mixture Is Smooth And Shiny. Stir In Gruyere And Mustard. Drop By Rounded Teaspoonfuls 2 In. Apart Onto Greased Baking Sheets. Sprinkle With Parmesan Cheese. Bake At 425