Add sugar, salt, unsalted butter, milk, and water to a pot.

Cook on low heat until it just starts to boil, then turn off the heat.

Sift in the cake flour and stir until it forms a dough.

Heat it again on low heat until a thin film forms at the bottom of the pot.

Transfer the dough to a bowl and let it cool slightly.

Add room temperature eggs in batches, mixing well after each addition until the egg mixture is no longer visible.

Check the batter consistency; it should form a triangle shape when it drips off the spatula. Do not add all the egg mixture if not needed.

Transfer the batter into a piping bag.

Pipe the batter in circles into two 6-inch molds.

Smooth the surface of the piped batter.

Bake at 180°C (356°F) for 30 minutes.

Let the choux pastry cool and set aside.

In a pot, add egg yolks, sugar, cornstarch, and cake flour, then mix well.

Add milk in batches and mix until fully combined.

Heat the mixture on low heat until it thickens.

Add unsalted butter and stir until incorporated.

Blend the custard using a hand blender to make it smoother and free of lumps.

Pour the custard into a bowl, cover with plastic wrap, and refrigerate until completely cool.

Whip the cream with sugar, using a whisk vertically to reduce air incorporation for a smoother texture.

Take out the chilled custard and whip it slightly until smooth.

Gradually add the whipped cream to the custard and mix until well combined.

In a 6-inch mold, place a layer of choux pastry at the bottom.

Add a layer of custard on top of the choux pastry and smooth the surface.

Add another layer of choux pastry on top of the custard.

Chill the assembled cake in the refrigerator for at least 4 hours, preferably overnight.

Dust the top with powdered sugar for decoration before serving.