Make the Pastry Cream (Do This First):

1. In a medium saucepan, heat milk over medium heat until it just begins to steam (don’t boil). Remove from heat.
2. In a mixing bowl, whisk together egg yolks and ¼ cup sugar until pale and thick, about 2 minutes.
3. Whisk cornstarch into the egg mixture until smooth and no lumps remain.
4. Slowly pour about ½ cup of hot milk into the egg mixture while whisking constantly (this is tempering). Then gradually whisk in the remaining hot milk.
5. Pour the entire mixture back into the saucepan. Cook over medium heat, whisking constantly, until it thickens and bubbles, about 3-5 minutes.
6. Remove from heat and whisk in butter, vanilla, and remaining ¼ cup sugar until smooth.
7. Pour through a fine-mesh strainer into a clean bowl to remove any lumps. Press plastic wrap directly onto the surface to prevent skin from forming. Refrigerate for at least 2 hours until completely cold.

Make the Choux Pastry:

8. Preheat your oven to 425°F (220°C). Line two baking sheets with parchment paper.
9. In a large saucepan, combine water, butter, and salt. Bring to a rolling boil over medium-high heat, ensuring butter is completely melted.
10. Remove from heat and immediately dump in all the flour at once. Stir vigorously with a wooden spoon until a smooth ball forms and pulls away from the sides of the pan.
11. Return pan to low heat and cook for 1-2 minutes, stirring constantly, to dry out the dough slightly. You’ll see a thin film form on the bottom of the pan.
12. Transfer dough to a mixing bowl and let cool for 5 minutes (it should still be warm, not hot).
13. Add eggs one at a time, beating well after each addition. The dough will look broken at first but will come together. After the final egg, the dough should be smooth, glossy, and hold a V-shape when you lift the spoon.
14. Transfer dough to a piping bag fitted with a large round tip. Pipe 4-inch long strips onto prepared baking sheets, spacing them 2 inches apart. You should get 12-14 éclairs.
15. Use a damp finger to gently smooth any peaks on top (they’ll burn otherwise).
16. Bake at 425°F for 15 minutes, then reduce heat to 375°F (190°C) and bake for another 20-25 minutes until deep golden brown and hollow-sounding when tapped. DO NOT open the oven door during baking or they’ll collapse.
17. Remove from oven and immediately poke a small hole in each end of each éclair with a skewer to release steam. Let cool completely on a wire rack.

Make the Chocolate Ganache:

18. Place chopped chocolate in a heatproof bowl. Heat cream in a small saucepan until it just begins to simmer.
19. Pour hot cream over chocolate and let sit for 1 minute, then whisk until smooth and glossy. Stir in corn syrup if using. Let cool for 10 minutes until slightly thickened but still pourable.

Assemble the Éclairs:

20. Once éclairs are completely cool, use a small round piping tip to poke a hole in both ends of each éclair (on the bottom).
21. Beat the cold pastry cream with a whisk until smooth. Transfer to a piping bag fitted with a small round tip.
22. Fill each éclair through one hole, stopping when cream starts coming out the other end. Wipe off excess.
23. Dip the top of each filled éclair into the chocolate ganache, letting excess drip off. Place on a wire rack and let chocolate set for 15-20 minutes.
24. Serve immediately or refrigerate for up to 4 hours before serving.