- Cut butter into small pieces and set it aside. Heat the oven to 400°F. Grease 2 large baking trays and sieve salt, flour, pinch of cayenne pepper and dry mustard.
- Boil water and butter in a saucepan. Add flour mixture in water, removing the pan from heat. Beat the mixture using a wooden spoon till it becomes like a ball. Place the saucepan back to the stove and heat it for at least 2 minutes. Remove and let it cool.
- Add 3 1/2 beaten eggs in dough, gradually mixing it. Make it sure that the dough does not become hard. Mix cheese and chives into it.
- Take a teaspoon full of dough and place them on baking tray, keeping a distance between each of them. Use another teaspoon if you are having difficulty to place the dough on baking tray. Mix a teaspoon of water with the remaining egg and brush beaten egg on top of every ball.
- Bake it for at least 8 minutes. Reducing the temperature to 350°F, bake it for another 8 minutes. Take them out and place them on a wire rack to cool down the puffs.