In a saucepan over medium-high heat, add milk, water, sugar, salt, and butter. Stir until butter is melted and the mixture is smooth.

Remove from heat and immediately add flour, stirring vigorously with a spatula until a smooth dough forms.

Return the pan to low heat and cook while stirring for 1 minute.

Transfer the dough to a bowl and let it cool for 5 minutes.

In a separate bowl, whisk eggs. Gradually add to the dough in 2-3 additions, mixing thoroughly each time.

Divide the batter into two greased and lined 7-inch cake pans. Use a spatula to create peaks for texture.

Bake in a preheated oven at 200°C/392°F for 25-30 minutes or until golden brown.

Let cool for 5-10 minutes, then transfer to a wire rack.

In a saucepan, heat milk, sugar, and vanilla extract until it comes to a boil. Set aside.

In another saucepan, whisk egg yolks and cornstarch until smooth.

Slowly pour in 1/3 of the hot milk while continuously stirring.

Add the remaining milk and mix well.

Cook over medium-low heat, stirring continuously, until the mixture thickens and comes to a boil.

Transfer to a bowl, cover with cling film (touching the surface), and let it cool to room temperature.

Beat softened butter until creamy, then gradually add the cooled custard while beating until smooth.

Line a 7-inch cake pan with parchment, ensuring overhang.

Place one choux layer at the bottom.

Spread the Crème Mousseline evenly on top.

Add the second choux layer, lightly press down, and cover loosely with cling film.

Refrigerate for 2-4 hours before slicing.

Dust with powdered sugar before serving.