Karpatka (Polish Carpathian Cream Cake)

To make the pâte à choux:

1. Preheat oven to 400˚F (200˚C). Grease two 10 inch (25.5 centimeter) round springform pans with butter, then line the bottom with parchment.
2. In a medium saucepan, place the water and 1 cup (227 grams) butter over medium heat.
3. In a medium bowl, combine the flour, baking powder, and salt.
4. Once the butter has completely melted, reduce heat to low and add the flour mixture all at once. Immediately beat with a wooden spoon or silicone spatula until fully incorporated and continue to stir until the dough lifts from the sides and bottom of the pan, about 2 minutes.
5. Remove the dough from heat and transfer to the bowl of a stand mixer with a paddle attachment or a large bowl. Allow to cool to room temperature.
6. Once the dough has cooled, beat the eggs into the dough, one at a time, until well-combined with a smooth and glossy texture.
7. Divide the dough into two equal pieces.
8. Place each piece in the prepared springform pans and spread the dough to cover the bottom, making an uneven texture with dents and waves across the surface. Do not smooth out the dough. These waves will create the mountainous shape as it bakes.
9. Bake both springform pans (I like to place them on baking sheets just in case any butter or moisture leaks out) in the preheated oven until puffed and golden, 20-25 minutes. Do not open the oven door until it is nearly done.
10. Turn off the oven and prop open the door slightly using the wooden handle of a spoon. Allow to rest for 10 minutes before removing from the oven and continuing to cool to room temperature.

To make the cream:

1. In a medium bowl, whisk the 5 egg yolks with 1/3 cup (66 milligrams) of the granulated sugar and the vanilla sugar until light and smooth. Set aside.
2. In another medium bowl, whisk together the potato starch and flour.
3. In a large saucepan, place the milk and remaining 1/3 cup (66 milligrams) granulated sugar over medium low heat.
4. Once steaming, slowly pour in a tablespoon (15 milliliters) of the heated milk into the egg mixture while continuing to whisk.
5. Once incorporated, slowly pour in another 1/2 cup (120 milliliters) of the heated milk while whisking the eggs. Repeat this process another 3 times (for a total of about 1 1/2 cups/355 milliliters of the milk) to temper the eggs.
6. Whisk the potato starch and flour mixture into the heated eggs.
7. Once incorporated, reduce heat to low and slowly pour the egg mixture back into the pot with the remaining milk while continuing to whisk.
8. Continue to stir, especially the bottom and sides of the pan, until the mixture has thickened to the consistency of pudding. Remove from heat and transfer to a heat-safe bowl.
9. Cover the top of the cream with plastic wrap, making sure it is directly touching the top, and allow to cool to room temperature.

To assemble the Karpatka:

1. In a large bowl, beat together the softened 14 tablespoons (200 grams) butter with the 2 tablespoons (25 grams) granulated sugar until light and fluffy.
2. Whisk the room temperature cream until smooth, then add a spoonful of the cream to the bowl with the light and fluffy butter mixture. Beat until incorporated, then repeat with another spoonful of cream. Continue to add cream until completely mixed into the butter.
3. Pick the best looking choux pastry and set it aside to use as the top of the cake.
4. Place the other pastry back in the bottom of the springform pan.
5. Pour the cream over the choux pastry in the pan in an even layer. Place the set-aside choux pastry over the cream, pressing very lightly to evenly seal.
6. Cover the cake in the springform pan with a layer of plastic wrap and refrigerate for at least 3 hours and up to overnight to set.
7. Remove the set cake from the refrigerator and gently remove from the springform pan.
8. Place the cake on the desired stand and cover with a layer of powdered sugar.
9. Cut into wedges and serve immediately.