Ingredients

5 large egg yolks

2/3 cup granulated white sugar (divided into 1/3 cup + 1/3 cup)

7 tablespoons potato starch

2 1/2 cups whole milk

1/4 teaspoon salt

2 teaspoons pure vanilla extract

2 teaspoons vanilla bean paste (or more vanilla extract)

200 grams unsalted butter (at room temperature, about 14 tablespoons + 1 teaspoon)

2 tablespoons granulated white sugar

3/4 cup water

6 tablespoons unsalted butter

1/8 teaspoon salt

3/4 cup all-purpose flour

3 large eggs

1 large egg yolk

Butter (to grease the pans)

Powdered sugar (for dusting on top)

Instructions

Step 1: Start the crème mousseline (crème pâtissière). In a medium bowl, beat the egg yolks with 1/3 cup sugar using a handheld mixer until thick and pale yellow. Mix in the potato starch until fully combined.

Step 2: In a medium saucepan, combine milk, remaining 1/3 cup sugar, and salt. Heat over medium until it gently simmers, then remove from heat.

Step 3: Gradually whisk about 1 cup of the hot milk into the egg yolk mixture, starting with just a few drops to temper. Pour this mixture back into the saucepan, whisking in the vanilla extract and vanilla bean paste.

Step 4: Return the pan to low heat and cook, whisking constantly, until the custard thickens, about one minute. Transfer to a bowl, cover directly with plastic wrap, and cool to room temperature.

Step 5: Prepare the choux pastry. Preheat the oven to 380°F. Line the bottoms of two 8 or 9-inch springform pans with parchment paper and butter the sides.

Step 6: In a medium saucepan over medium heat, combine water, butter, and salt. When butter melts, remove from heat and stir in all flour at once. Return to low heat and stir constantly until dough forms a ball and leaves residue on the pan’s bottom, about 2 minutes.

Step 7: Transfer the dough to a large bowl and let cool for 5 to 10 minutes until warm but not hot. Beat in the eggs and extra egg yolk one at a time with a handheld mixer until smooth and glossy.

Step 8: Divide dough between the two pans. Spread evenly to cover the bottoms, then use a spoon’s back to create peaks and divots for a mountain-like texture.

Step 9: Bake for 25 to 28 minutes until puffed and golden. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Step 10: Finish the crème mousseline by beating the room-temperature butter and 2 tablespoons sugar until pale and fluffy. Gradually beat in the cooled custard, one spoonful at a time, until fully combined and light, about 5 minutes.

Step 11: If the choux cakes aren’t fully cool yet, cover the mousse with plastic wrap directly on the surface to prevent drying.

Step 12: To assemble, place one choux cake in a springform pan and spread the crème mousseline evenly over it. Top with the second cake.

Step 13: Refrigerate for at least 1 hour before serving. Remove from the pan and dust with powdered sugar just before serving.

Tips & Variations

Use vanilla bean paste for the most authentic, intense vanilla flavor, but pure vanilla extract works well too.

To create a more dramatic mountain effect, spread the choux dough less evenly to encourage higher peaks.

Ensure the custard is completely cool before mixing with butter to avoid melting the butter and ruining the texture.

For a lighter filling, fold in whipped cream to the crème mousseline before assembling.

FAQs

Can I make Karpatka ahead of time?

Yes, you can prepare the cake a day in advance and refrigerate it to let the flavors meld. Just dust with powdered sugar before serving for a fresh appearance.