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

Place the water, butter, and salt in a saucepan over medium heat. Bring the mixture to a full boil and make sure 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 and pulls away from the sides of the pan.

Return the pan to medium-low heat and stir for 1–2 minutes to dry the dough slightly. Look for a thin film forming on the bottom of the pan.

Transfer the dough to a mixing bowl and cool it for about 5 minutes. Beat in the eggs one at a time, mixing well after each addition, until the dough is smooth, glossy, and thick enough to hold its shape.

Pipe or spoon 12 mounds of dough, each about 2 inches (5 cm) wide, onto the prepared baking sheet. Smooth any pointed tops with a damp fingertip.

Bake at 425°F (220°C) for 10 minutes without opening the oven. Reduce the temperature to 375°F (190°C) and bake for another 18–25 minutes, until the puffs are deeply browned and feel light and dry.

Turn off the oven, crack the door open, and leave the puffs inside for 5–10 minutes. Transfer them to a rack, cool completely, and split them horizontally or cut a small opening in each puff.

Beat the cold heavy cream, confectioners’ sugar, and vanilla extract until medium-stiff peaks form. Stop mixing as soon as the cream holds its shape so it does not become grainy.

Toss the berries with 1–2 tablespoons of confectioners’ sugar if desired. Fill each cooled puff with vanilla cream and berries, replace the tops, and dust with additional confectioners’ sugar before serving.