Add the egg yolks and 1/3 cup sugar to a medium bowl. Use a handheld electric mixer to beat together until the mixture is thick and pale yellow. Beat in the potato starch until well-combined.

Add the milk, 1/3 cup sugar, and salt to a medium saucepan over medium heat. Bring to a gentle simmer.

Remove the milk mixture from the heat. Gradually, starting with just a couple drops at first, whisk 1 cup of the hot milk into the egg yolk mixture. Pour the egg yolk mixture into the pot and whisk in the vanilla extract and vanilla bean paste.

Return the saucepan to the heat, and turn the heat down to low. Whisking constantly, cook until the custard is thickened. This should take less than a minute.

Once thickened, transfer the custard to a bowl and place a piece of plastic wrap directly on top. Cool to room temperature.

Preheat the oven to 380F. Line the bottom of 2 (8 or 9-inch) springform pans with parchment paper that’s trimmed to fit inside. Grease the sides with butter.

Add the water, butter, and salt to a medium saucepan over medium heat. Once the butter is melted, remove the pan from the heat and stir in the flour all at once. Turn the heat down to low and return the pan to the heat. Cook until the dough forms a ball and leaves a residue on the bottom, about 2 minutes, stirring constantly.

Transfer the dough to a large bowl and let it cool until it’s still quite warm, but not too hot to touch, about 5 to 10 minutes. Use a handheld electric mixer to beat in the eggs and egg yolk 1 at a time until fully incorporated. The resulting batter will be smooth and glossy.

Divide the dough between the 2 prepared pans. Spread the dough out to cover the bottom of each pan, and then use the back of a spoon to spread it unevenly to create divots and peaks (this is so the finished cake looks like the Carpathian Mountains).

Bake until the choux cake is puffed and evenly golden, about 25 to 28 minutes.

Cool for 10 minutes in the pans, then remove the cakes from the pans and cool completely on a wire rack.

Once the custard is cool, use a handheld electric mixer to cream together the butter and 2 tablespoons sugar in a large bowl until pale and fluffy.

Add a spoonful of vanilla custard at a time and beat until incorporated, continuing until all the custard is mixed in. Continue beating until the mixture is light and fluffy, about 5 minutes.

Place a piece of plastic wrap directly on top of the crème mousseline if the cakes aren’t completely cool yet.

Refrigerate 1 hour before serving, and then remove it from the springform pan. Right before serving, dust the top with powdered sugar.