Prepare the choux pastry by combining water and unsalted butter in a saucepan, bringing to a boil, and then stirring in flour until a ball forms. Add beaten eggs one at a time.

Preheat oven to 400°F (200°C). Spread the choux pastry on a parchment-lined tray to form two layers. Bake for 25-30 minutes until golden brown.

Make the vanilla custard by whisking egg yolks, sugar, and cornstarch, then gradually adding milk and vanilla over medium heat until thickened.

Once baked, cool the choux pastry layers for about 30 minutes on a wire rack.

Assemble the cake by placing one pastry layer on a platter, spreading custard over it, and topping with the second pastry layer.

Refrigerate the assembled cake for at least 2 hours before serving to allow flavors to meld.