1. Preheat the oven to 200°C. Grease the bottom and sides of two 9-inch springform pans with butter and line the bottoms with greaseproof paper. If you only have one pan, you will need to bake one layer of the cake and then prepare the pan again to make the second layer.
2. Put the butter, water and salt in a heavy-based saucepan and bring to the boil. Add the flour and stir well with a wooden spoon to ensure that no lumps form. Cook over a medium heat, stirring for 5 minutes until the mixture comes away from the sides of the pan and forms a smooth ball. Transfer dough to a stand mixer and beat at medium edium speed for 2 minutes. Add the eggs in one at a time, mixing well between each addition until the mixture is smooth.
3. Split the dough into two even parts and spread between the two spring form pans. Bake for 25-30 minutes or until an even golden brown colour is achieved. Remove from the oven and leave to cool down for 10 minutes before removing it from the pan onto a cooling rack to cool completely.
4. For the crème mousseline, place the milk and cut vanilla pod in a pan and bring to the boil. Whisk together the yolks and sugar in a mixing bowl to a light ribbon consistency. Add the flour and whisk it thoroughly. Slowly add the milk to the egg mixture in stages while whisking the mixture together. Pour the mixture back into the pan and cook the mixture on a medium heat for 5 minutes, until the flour is cooked and the mixture is thick.
5. Remove from the heat and pour into the bowl of a stand mixer. Beat on a medium speed until it reaches room temperature, ensuring you scrape the sides of the bowl. Increase the speed, then gradually add the butter cube by cube, until it is fully combined.
6. To assemble, line the side of your springform pan with a strip of greaseproof paper. Place one of the choux pastry layers inside the ring. Spread the Plum & Bay Leaf Jam evenly over the base followed by the crème mousseline. Top with the remaining layer of choux pastry. Refrigerate Karpatka for at least 3 hours to allow the filling to set. Remove the ring and greaseproof paper before dusting the top with icing sugar and serving
