Heat the oven to 425°F (220°C) and line a baking sheet with parchment paper.

Combine the water, butter, sugar, and salt in a saucepan. Bring the mixture to a full boil over medium heat, making certain that the butter has completely melted.

Remove the saucepan from the heat and add all of the flour at once. Stir vigorously until a smooth dough forms.

Return the pan to medium-low heat and cook the dough for 1–2 minutes, stirring constantly, until it pulls away from the pan and leaves a thin film on the bottom.

Transfer the dough to a mixing bowl and cool it for 5–10 minutes. Add the eggs one at a time, beating thoroughly after each addition; add the final egg gradually if the dough already looks glossy and soft.

Pipe or spoon the dough into 1 1/2- to 2-inch mounds on the prepared sheet, leaving room between them. Smooth any pointed tops with a damp fingertip.

Bake at 425°F for 10–15 minutes without opening the oven. Reduce the temperature to 350°F (175°C) and bake for another 20–30 minutes, until deeply golden and firm.

Turn off the oven, crack the door open, and leave the puffs inside for about 10 minutes. Transfer them to a rack, prick each once or twice if desired, and cool completely.

Whip the cold heavy cream, confectioners’ sugar, and vanilla to medium-stiff peaks. Slice each cooled puff horizontally or make a small opening and pipe in the filling.

Dust the cream puffs with powdered sugar or drizzle them with melted chocolate or ganache. Fill shortly before serving for the crispest shells.