In a medium saucepan over medium heat, combine 1 cup of water, 1/2 cup of unsalted butter, and 1/4 teaspoon of salt. Bring to a boil, then add 1 cup of flour and stir until a dough forms.

Allow the dough to cool slightly, then incorporate 4 eggs one at a time until the dough is smooth and glossy.

Preheat oven to 400°F (200°C), then pipe the choux pastry onto baking sheets and bake for 30-35 minutes until golden brown.

In a saucepan, combine 2 cups of whole milk, 1/2 cup of granulated sugar, and a pinch of salt. Heat until simmering.

Whisk together 4 egg yolks, 1/3 cup of sugar, and 3 tablespoons of potato starch. Gradually add the hot milk.

Return the mixture to the pan and whisk until thickened. Remove from heat, then stir in vanilla extract and butter until smooth.

Layer the first choux pastry in a springform pan, add the crème mousseline, and top with the second choux layer. Dust with powdered sugar before serving.