Preheat the oven to 425°F and line a baking sheet with parchment paper. In a medium saucepan, bring the water, unsalted butter, and salt to a rolling boil. Add the flour all at once and stir vigorously with a wooden spoon. Continue stirring until the dough forms a smooth ball and pulls away from the sides of the pan. Remove the saucepan from the heat and let the dough cool for 3–5 minutes. Add the eggs one at a time, beating well after each addition until the dough becomes smooth and glossy. The dough should be thick enough to hold its shape when spooned onto the baking sheet.

Drop rounded tablespoons of dough onto the prepared baking sheet, leaving enough space between each mound for the pastry to expand. Bake for 20–25 minutes, until the cream puffs are well risen and golden. Do not open the oven during baking, because a sudden drop in temperature can cause the shells to collapse. Turn off the oven, crack the door slightly, and leave the shells inside for 10 minutes. Transfer them to a wire rack and allow them to cool completely before filling.

Whisk the instant vanilla pudding mix with the cold milk until thickened. In a separate bowl, whip the heavy cream with the vanilla extract until stiff peaks form. Gently fold the whipped cream into the pudding mixture until evenly combined. Slice each cooled shell in half or make a small hole in the side or bottom. Spoon or pipe the vanilla cream into the center, adding enough filling to make each puff generous without forcing the pastry apart.

Arrange the filled cream puffs on a serving plate. Dust the tops with powdered sugar for a classic finish, or add a drizzle of melted chocolate if you prefer a richer dessert. Serve shortly after filling for the best contrast between the delicate pastry and creamy center. If preparing ahead, keep the shells and filling separate until closer to serving time.