Crème Mousseline:

In a medium-sized mixing bowl, beat the egg yolks with ⅓ cup of sugar until the mixture becomes pale and fluffy.

Add potato starch to the egg yolk mixture and whisk until well combined.

In a medium-sized saucepan, heat the milk with the remaining ⅓ cup of sugar and salt over medium heat until it starts to bubble around the edges.

Slowly pour the hot milk mixture into the egg yolk mixture, whisking constantly to prevent the eggs from curdling.

Pour the mixture back into the saucepan and cook over medium-low heat, stirring constantly, until it thickens and comes to a boil.

Remove the saucepan from the heat and stir in the vanilla extract and vanilla bean paste.

Transfer the pastry cream to a clean bowl, cover with plastic wrap pressed directly onto the surface of the cream, and refrigerate until completely chilled.

In a separate mixing bowl, beat the softened butter with 2 tablespoons of sugar until light and fluffy.

Add the chilled pastry cream to the butter mixture and beat until well combined and smooth.

Cover the bowl with plastic wrap and refrigerate until ready to use.

Choux Pastry:

Preheat your oven to 400°F (200°C) and grease two 9-inch round springform pans with butter.

In a medium-sized saucepan, bring the water, butter, and salt to a boil over medium heat.

Remove the saucepan from the heat and quickly stir in the flour until the mixture forms a smooth ball.

Return the saucepan to low heat and continue to cook, stirring constantly, for about 1-2 minutes to dry out the dough.

Transfer the dough to a mixing bowl and let it cool for 3-5 minutes.

Add the eggs, one at a time, beating well after each addition. Finally, beat in the egg yolk.

Divide the choux pastry evenly between the prepared pans and smooth the tops.

Bake in the preheated oven for 35-40 minutes, or until the pastries are puffed up and golden brown.

Remove the pans from the oven and let the pastries cool completely in the pans.

Assembly:

Remove the choux pastries from the pans.

Using a serrated knife, gently slice each pastry in half horizontally to create two layers.

Place one layer of choux pastry on a serving plate.

Spread a generous amount of the chilled crème mousseline over the pastry layer.

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

Dust the top of the cake with powdered sugar.

Serve the Karpatka immediately or refrigerate until ready to serve.