Preheat your oven to 400°F (200°C). In a saucepan, bring water and butter to a boil.

Add flour all at once, stirring vigorously until a smooth ball forms.

Remove from heat and let cool for 5 minutes.

Add eggs, one at a time, beating well after each addition until the mixture is smooth.

Spread the dough evenly into two greased 9-inch round cake pans. Bake for 30-35 minutes or until golden brown and puffed.

Remove from oven and cool completely on wire racks.

In a saucepan, heat the milk over medium heat until steaming but not boiling.

In a separate bowl, whisk together sugar, cornstarch, and egg yolks until smooth and pale.

Gradually whisk the hot milk into the egg mixture.

Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thickened.

Remove from heat and stir in vanilla extract. Transfer to a bowl, cover with plastic wrap (directly touching the surface of the custard to prevent a skin from forming), and chill in the refrigerator until completely cold.

In a separate bowl, whip the chilled heavy cream until stiff peaks form. Gently fold the whipped cream into the chilled custard until well combined.

Carefully remove the pastry layers from the cake pans.

Place one layer on a serving plate and spread the cream filling evenly over the top.

Place the second pastry layer on top of the cream filling.

Dust with powdered sugar if desired.

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

Slice and serve chilled, savoring each bite of the light, airy pastry and creamy filling.