Combine the butter, sugar, salt and water in a heavy bottomed saucepan over medium high heat.
Once the butter is melted and it starts to boil, remove from heat and stir in the flour with a with a wooden spoon or heat proof spatula. When it is combined place back over heat.
Stir the mixture constantly of medium high for about 4 minutes. You are trying to dry it out a bit and cook the flour. It will start to pull away from the edges and a film will form on the bottom of the pan.
Transfer the batter to a standing mixer and turn on low. Mix for about 2 minutes to cool it down.
While still mixing on slow, add 4 eggs one at a time. Make sure to fully incorporate each egg before you add the next.
Test the choux pastry batter to see if it’s the right consistency by touching it with your finger or a spatula. It should pull away in a string. If it doesn’t, mix up another egg and slowly add it to mixture one teaspoon at a time until it is the right consistency.
Fill your pastry bag with batter and pipe onto the baking sheets. You can either pipe one sheet and store the reminder of the pastry cream in the piping bag in the fridge until you're ready to bake the second tray. Or, you can pipe out the second tray at the same time and store it with a lightly oiled plastic wrap in the fridge. I normally just keep it it in the piping bag and pipe right before I’m going to cook
If using craquelin, place on top of each puff. If not, mix together egg and water and using a pastry brush or your finger gently rub the egg wash over the entire surface, patting down any pointed tips.
Bake for 15 minutes at 400 degrees and then turn down the temperature to 350 degrees and bake 20 more minutes. Do not open the oven while they are baking. That will release the steam and inhibit them rising to their full potential.
Once they are golden brown you can remove from oven and let them cool and transfer to wire rack. Return oven to 400 degrees and repeat for the remainder of the pastry batter.
Once they are cooled you can fill with your filling of choice and enjoy!