Karpatka Cake Recipe with Mousseline Cream

Instructions:

1. Combine 250ml water and 115g butter in a pot until bubbling and butter is melted.
2. Dump in 140g flour and 1/4 tsp salt all at once, stirring vigorously until a smooth ball forms. Keep cooking for 2 mins to cook off the raw flour taste.
3. Transfer dough to a bowl; add 5 eggs one by one until the dough is glossy and hangs in a 'V' shape.
4. Spread half the dough into a lined 23cm square pan and bake at 200°C for 25 mins until deeply golden and craggy.
5. Cool completely. Repeat for the second half of the dough. Never stack these while warm or they will go soft.
6. Mix 250ml of the milk with 85g cornstarch and 2 yolks until no lumps remain.
7. Heat remaining 500ml milk with 150g sugar and vanilla; stir in cornstarch mix until thick like heavy pudding.
8. Whip 225g softened butter until pale and airy.
9. Add the cooled custard to the butter one spoon at a time until a thick, velvety cream forms.
10. Spread all the cream over one pastry layer, top with the second, and chill for 4 hours until the filling is set.