Step 1: Make the Pastry Cream

1. Whisk together egg yolks and sugar in a large bowl until light yellow and smooth.
2. Add corn starch and whisk until thick.
3. In a saucepan, heat milk and vanilla over medium-low heat until steaming (do not simmer).
4. Slowly pour the warmed milk into the egg mixture, whisking continuously.
5. Strain the mixture back into the pot and whisk continuously for 5-7 minutes until thick.
6. Remove from heat and stir in butter until smooth.
7. Cover with plastic wrap (directly on the surface) and let cool for 4-5 hours.

Step 2: Make the Choux Pastry

8. Preheat oven to 375°F (190°C).
9. In a saucepan, heat butter, water, sugar, and salt until the butter melts and the mixture simmers.
10. Add flour all at once, stirring vigorously until a smooth dough forms and pulls away from the sides.
11. Transfer to a bowl and let cool for 5 minutes.
12. Using a hand or stand mixer, beat in eggs one at a time until the dough becomes smooth, shiny, and thick.

Step 3: Bake the Éclair Layers

13. Line two springform pans with parchment paper and butter the sides.
14. Divide the dough evenly between the pans, spreading it out while creating small peaks for texture.
15. Bake for 28-30 minutes until golden brown and puffy.
16. Let cool for 5 minutes, then remove and transfer to a wire rack.

Step 4: Prepare the Filling

17. Whip heavy cream and powdered sugar until stiff peaks form.
18. Beat the chilled pastry cream until smooth, then fold in the whipped cream in thirds.

Step 5: Assemble the Cake

19. Line the edges of the springform pan with parchment paper for easy release.
20. Place one éclair layer at the bottom, spread all the diplomat cream filling, and top with the second éclair layer.
21. Cover with plastic wrap and refrigerate for at least 2 hours before slicing.
22. Serve chilled and enjoy!