Make the Pastry Cream First (it needs to chill):

1. Heat the milk: In a medium saucepan, heat the milk and ¼ cup sugar over medium heat until it just begins to simmer. Remove from heat.
2. Whisk the egg mixture: In a mixing bowl, whisk together egg yolks, remaining ¼ cup sugar, and cornstarch until smooth and pale yellow.
3. Temper the eggs: Slowly pour about ½ cup of the hot milk into the egg mixture while whisking constantly. This prevents the eggs from scrambling.
4. Cook the cream: Pour the tempered egg mixture back into the saucepan with the remaining milk. Cook over medium heat, whisking constantly, until the mixture thickens and comes to a boil (about 2-3 minutes).
5. Finish and chill: Remove from heat, whisk in butter and vanilla until smooth. Strain through a fine-mesh strainer into a bowl. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate for at least 2 hours or until completely chilled.

Make the Choux Pastry:

6. Preheat oven: Preheat your oven to 425°F (220°C). Line two baking sheets with parchment paper.
7. Make the dough: In a medium saucepan, combine water, butter, sugar, and salt. Bring to a boil over medium-high heat, stirring occasionally until butter melts completely.
8. Add flour: Remove from heat and add all the flour at once. Stir vigorously with a wooden spoon until a smooth dough forms and pulls away from the sides of the pan.
9. Cook the dough: Return the pan to medium heat and cook for 2 minutes, stirring constantly, to dry out the dough slightly. You’ll see a thin film form on the bottom of the pan – this is perfect!
10. Transfer and cool: Transfer the dough to a mixing bowl (or the bowl of a stand mixer) and let it cool for 5 minutes. This prevents the eggs from cooking when you add them.
11. Add eggs: Beat in the eggs one at a time, mixing well after each addition. The dough will look broken at first but will come together into a smooth, glossy, pipeable consistency. You want the dough to fall slowly off the spoon in a thick ribbon.
12. Pipe the puffs: Transfer the dough to a piping bag fitted with a ½-inch round tip. Pipe 1½-inch mounds onto the prepared baking sheets, spacing them 2 inches apart (they will expand!). Wet your finger and gently press down any peaks so they don’t burn.
13. Bake: Bake at 425°F for 15 minutes, then reduce temperature to 350°F (175°C) and bake for another 20 minutes until golden brown and crispy. DO NOT open the oven door during baking – the puffs will deflate!
14. Create vents: Remove from oven and immediately poke a small hole in the side of each puff with a toothpick to release steam. This keeps them crispy. Let cool completely on a wire rack.

Fill the Cream Puffs:

15. Prepare filling: If using whipped cream, beat 1 cup heavy cream to stiff peaks and fold into the chilled pastry cream for a lighter texture. Or use the pastry cream as-is for richer filling.
16. Fill the puffs: Cut each puff in half horizontally, or make a small hole in the bottom and pipe filling inside. Use a piping bag fitted with a small round tip, or simply spoon the cream into split puffs.
17. Finish and serve: Dust with powdered sugar, drizzle with chocolate sauce, or serve plain. Best enjoyed within a few hours of filling.