Position the oven racks to the top and bottom thirds of the oven and preheat to 400 degrees.
Make templates for your puffs or eclairs on parchment paper, turn the parchment over so the writing doesn't transfer to the dough, and place on (2) baking sheets.*
Place the water, butter, and salt in a medium saucepan and heat over low heat until the butter has melted completely. Increase the heat to medium and bring to a full boil. Remove from heat and add the flour all at once, stirring vigorously with a wooden spoon until incorporated and the dough forms a cohesive mass around the spoon.
Place back over medium heat and cook, stirring constantly, for 1-2 minutes, until the dough leaves a thin film on the bottom of the pan.
Place the dough in the bowl of a stand mixer fitted with the paddle attachment and mix on medium speed for 1-2 minutes or until the dough has cooled slightly.
With the mixer running constantly, add the egg about 2 tablespoons at a time, making sure it's fully incorporated before adding more. Repeat until all of the egg has been mixed in.
If making puffs, pipe or spoon the dough onto the templates -- if eclairs, definitely pipe it. Smooth out any pointed tips with your finger or lightly oiled butter knife. I used a 1/2" round piping tip.
Bake at 400 degrees for 20 minutes. Reduce the heat to 350 degrees, rotate the pans from top to bottom and front to back, and bake an additional 20 minutes. Turn off the oven. Remove the baking sheets from the oven and cut small slits toward the bottom of the pastry on both sides. Place back in the turned off oven to dry out for an additional 10 minutes.
Fill with pastry cream, whipped cream, mousse, or savory foods like chicken salad or spinach dip. Enjoy!