Step 1 Place the water or milk, butter, and salt in a medium saucepan. Place over high heat and bring to a full rolling boil.
Step 2 Remove from the heat and add the flour all at once. Using a wooden spoon or stiff silicone spatula and stir until the flour is thoroughly mixed in and no lumps remain, pressing and smashing the dough against the sides of the pan.
Step 3 Return the pot to medium-high heat and cook, stirring frequently, until the dough comes together into one mass and there is a dry film on the bottom and sides of the pan. If you have an instant-read thermometer, it should register 175 degrees F.
Step 4 Transfer the dough to a mixing bowl and beat on medium speed for about 2 minutes to cool the dough down. It should register 145 degrees on an instant-read thermometer.
Step 5 Add the eggs one at a time, ensuring each one is fully incorporated before adding the next. Once all of the eggs have been added, scrape down the bottom and sides of the bowl and mix again to ensure everything is fully incorporated, an additional 5 to 10 seconds.
Step 6 Use immediately or keep at room temperature for up to 2 hours in a piping bag or with a piece of plastic wrap pressed against the surface.
