Karpatka – Polish Vanilla Custard Cake

Line a rectangular baking pan or small baking tray measuring about 23 x 33 cm with parchment paper. Preheat the oven to 180°C fan or 200°C top/bottom heat.

For the choux pastry, place 250 ml water and 125 g butter in a saucepan. Heat until the butter has completely melted and the liquid reaches a vigorous boil.

Meanwhile, sift the flour and baking powder into a bowl and combine well. Add the mixture to the boiling liquid all at once and immediately stir firmly with a sturdy wooden spoon. Continue stirring over medium heat for 1 to 2 minutes, until a smooth ball of dough forms, pulls away from the sides of the pan, and leaves a pale film on the bottom.

Transfer the choux pastry to a mixing bowl and leave it to cool for about 10 minutes. It should be only slightly warm before adding the eggs. Beat in three eggs one at a time with the dough hooks or beaters of a hand mixer, fully incorporating each egg before adding the next. Beat the fourth egg in a small bowl and add it gradually, using only as much as needed for a smooth, glossy dough. When the beater is lifted, the dough should fall slowly in a broad, thick ribbon rather than breaking off firmly or running like a liquid.

Spread half of the choux pastry in the prepared pan. Keep the surface intentionally uneven so the characteristic Karpatka peaks and waves form while baking. Bake for about 20 minutes until golden brown. Do not open the oven door during baking, or the pastry may collapse.

Carefully remove the baked layer from the pan and let it cool completely on a wire rack. Repeat with the remaining half of the pastry.

While the pastry cools, bring 300 ml of the milk to a boil in a saucepan. Mix the remaining 200 ml milk with the vanilla pudding powder, vanilla sugar, and 40 g sugar until smooth. Slowly pour this mixture into the boiling milk while whisking continuously. Bring it briefly back to a boil until a very thick, smooth vanilla pudding forms.

Transfer the pudding to a clean bowl and press plastic wrap directly onto the surface to prevent a skin from forming. Let it cool completely to room temperature.

Beat the softened butter with a hand mixer for several minutes until pale and fluffy. The butter and pudding should be at roughly the same temperature. Add the room-temperature pudding to the butter one tablespoon at a time, mixing each portion in completely before adding the next, until the vanilla cream is smooth and stable.

Place one choux pastry layer on a serving plate or back in the cleaned pan. Spread the vanilla cream evenly over it, then set the second pastry layer on top with the more deeply rippled side facing upward.

Chill the Karpatka for at least 2 hours so the filling can set. Before serving, dust generously with powdered sugar. If desired, lightly toast the sliced almonds in a dry pan, let them cool, and scatter them over the cake.