Step 1: Preheat your oven to 400°F (200°C). Prepare two baking sheets by lining them with parchment paper or silicone mats.
Step 2: In a medium saucepan, combine the water, butter, sugar, and salt. Place over medium heat and bring the mixture to a rolling boil. Immediately remove the saucepan from the heat. Add all of the flour at once and stir vigorously with a wooden spoon until a smooth ball of dough forms.
Step 3: Return the saucepan to low heat. Continue to cook and stir the dough constantly for 1-2 minutes. This crucial step helps to dry out the dough, which is essential for airy puffs. A thin film should form on the bottom of the pan. Remove from heat and transfer the dough to a large mixing bowl. Allow it to cool for about 5 minutes.
Step 4: Add the eggs one at a time to the cooled dough, beating thoroughly after each addition until each egg is fully incorporated and the dough is smooth and glossy. The final dough should be thick enough to hold its shape when piped.
Step 5: Transfer the choux pastry to a pastry bag fitted with a large round tip. Alternatively, you can use a sturdy Ziploc bag with a corner snipped off. Pipe 1.5-2 inch mounds onto your prepared baking sheets, ensuring they are spaced about 2 inches apart.
Step 6: Bake the puffs for 20 minutes at 400°F (200°C). After 20 minutes, reduce the oven temperature to 350°F (175°C) and continue baking for another 15-20 minutes, or until the puffs are deeply golden brown and feel hollow. Turn off the oven, prop the door open slightly, and let the puffs cool inside for 10-15 minutes to prevent them from deflating. Transfer them to a wire rack to cool completely.
Step 7: To make the custard, whisk together the whole milk and half of the granulated sugar in a clean saucepan over medium heat. Stir occasionally and bring the mixture to a gentle simmer.
Step 8: In a separate bowl, whisk together the egg yolks, remaining sugar, and cornstarch until the mixture is smooth and pale.
Step 9: Gradually temper the hot milk mixture into the egg yolk mixture, whisking constantly to prevent the eggs from scrambling. Pour the entire mixture back into the saucepan and cook over medium heat, whisking continuously, until the custard thickens significantly and comes to a boil. Boil for 1 minute, still whisking, to ensure proper cooking.
Step 10: Remove the custard from the heat. Stir in the vanilla extract and softened unsalted butter until the custard is smooth and creamy. Transfer the custard to a clean bowl, cover the surface directly with plastic wrap (to prevent a skin from forming), and refrigerate until completely chilled, ideally for at least 2 hours or overnight.
Step 11: Once the custard is thoroughly chilled, whisk it briefly to loosen its consistency. If desired, transfer it to a pastry bag with a small round tip. Make a small hole in the bottom or side of each cooled cream puff and pipe in the custard until fully filled.
Step 12: Arrange the filled cream puffs on a serving platter. Generously dust them with powdered sugar just before serving. Enjoy!