Preheat oven to 400°F.
Add the water, butter and salt to a medium pot. Heat on medium high to bring to a boil.
6 ounces water, 6 tablespoons unsalted butter, 1/4 teaspoon salt
Stir in the flour all at once using a wood or silicone spoon or spatula. Continue to cook, stirring, until the mixture forms into once mass that pulls away from the sides of the pot and leaves a film on the bottom of the pot. The dough should be about 175°F at this point.
3.38 ounces all purpose flour
Transfer the dough to the mixer bowl and turn onto medium speed for about 1 minute. The temperature of the dough needs to drop to 145ºF or less before adding the eggs to prevent the eggs from cooking. You can check the temperature with a food thermometer if you have one. Add the egg white to the bowl and continue mixing until it’s fully incorporated, then add the whole eggs one at a time until each of those is fully incorporated. The dough may get lumpy as you add the eggs but will smooth out fairly quickly.
1 egg white, 3 eggs
Add the cheese and continue to mix for another minute, then turn off the mixer.
2.5 ounces Gruyere cheese
Transfer the dough to the prepared pastry bag.
Pipe small mounds of the batter out onto a parchment paper lined baking sheet. The mounds can be any size, but traditionally are about 1 inch in diameter and about 1 inch high.
Smooth off any points of batter with a wet finger so that they don’t burn while baking.
Optional: you can sprinkle Parmesan cheese on top prior to baking.
Bake for 15-18 minutes, or until golden brown. You can leave them in longer if a crispier crust is desired. Serve warm or at room temperature.