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 inches apart onto greased baking sheets. Sprinkle with Parmesan cheese. Bake at 425° for 15-20 minutes or until golden brown. Serve warm or cold. Yield: 4 dozen.