Preheat the oven to 200C/390F (top and bottom heat, no fan). Line the bottoms of two 23cm/9-inch springform pans with parchment paper and grease the sides with butter or cooking spray.

In a saucepan, combine water, milk, unsalted butter, sugar, salt, and vanilla extract, then heat over medium heat until it starts simmering. Remove from the heat and add the sifted flour. Mix until no dry flour is left, bring back to the heat, and set it to low.

Cook the dough, mixing with a rubber spatula continuously for 2 minutes or until the dough forms a ball and the bottom of the saucepan is covered with white residue. Remove from the heat.

Place the dough into a large mixing bowl or the bowl of a stand mixer fitted with a paddle attachment. Let it cool down for 10 minutes or until it’s no longer hot to touch but still very warm.

Start by adding the eggs a little at a time and mix until they are fully incorporated. Scrape the sides and the bottom of the bowl and beat one last time for just a few seconds to ensure thorough mixing. The dough should be smooth.

Split the dough into two even parts and spread unevenly between two prepared spring forms. Bake for 25-27 minutes or until reaching a deep, even golden-brown color on top. Remove from the oven and leave to cool down for 10 minutes before removing it from the form onto a cooling rack to cool down completely.

Place milk, half of the sugar, and salt into a saucepan and heat over medium heat until simmering. Meanwhile, combine the second half of the sugar with the cornstarch in a heatproof mixing bowl, then add the egg yolks and vanilla. Whisk until a homogenous consistency is reached without cornstarch lumps.

When the milk starts simmering, remove from the heat and slowly add approximately ½ of it to the egg yolk mixture while whisking vigorously to temper the eggs. Then, pour the egg mixture back into a saucepan with the hot milk.

Cook over low heat for 5-9 minutes, whisking consistently until the mixture becomes pudding-like in consistency, then cook for about 2 minutes. Remove from the heat and pour the mixture into a large, shallow dish. Cover the pastry cream with plastic wrap so they touch each other and let it cool down to room temperature.

In a stand mixer fitted with a whisk attachment, beat the butter at room temperature with 15g of sugar until white and fluffy. Whisk the pastry cream until it becomes smooth again.

Add the pastry cream a couple of tablespoons at a time to the butter and whisk every time on medium until incorporated. Continue until all pastry cream is combined with the butter.

Assemble your Karpatka cake and chill. Then, dust with powdered sugar and serve.