In a saucepan, bring to boil a milk over medium heat.

In a medium bowl, add eggs and sugar, then mix to combined. Add cornstarch then mix again until there is no lump. Gradually pour the hot milk into the egg mixture until well combined. Return to heat. Stirring constantly until thickens. Add vanilla extract and mix well.

Cover it with plastic wrap and press it directly onto the surface of the mousseline cream to prevent it from forming a skin on the surface. Let it cool completely.

Preheat the oven at 375°F or 190℃.

In a saucepan combine milk, water, butter, sugar, and salt. Bring to a boil. Then turn off the heat.

Add flour and stir quickly until well combined. Return to heat again and cook for one-two minutes more or until the dough cleans sides of the saucepan.

Transfer the dough to a mixing bowl and let it cool slightly. Add the eggs one at a time and stir vigorously with wooden spoon until uniform and shiny.

Divide evenly the choux dough into two then pour it to a cake pan that is greased and lined with a parchment paper. Bake for 30 minutes. Let it cool.

Whisk the butter over medium speed until creamy then add cooled pastry cream then whisk again until well combined.

Return one of the choux layer to a cake pan then pour the mousseline cream. Fill the gaps of the second choux layer with a bit of mousseline cream then place it to the cake pan. Refrigerate for 2-4 hours.

Dust with powdered sugar and the karpatka is ready to serve. Enjoy!