1. In a large saucepan, bring water, butter, salt and cayenne to a boil. Add the flour all at once and stir until a smooth ball forms. Remove from heat; let stand for 5 minutes. Add the eggs, one at a time, beating well after each addition. Continue beating until mixture is smooth and shiny. Stir in Gruyere and mustard.
2. Drop by rounded teaspoonfuls 2 in. apart onto greased baking sheets. Sprinkly with Parmesan cheese. Bake at 425 for 15-20 minutes or until golden brown. Serve warm or cold.