Mix butter, sugar and salt until light and creamy.
Add eggs one by one.
Mix in, flour and baking powder.
Spread evenly on a baking tray with baking paper.
Wash Rhubarb thoroughly and cut in pieces.
Spread Rhubarb pieces on dough, pressing lightly.
Bake in preheated oven at 170° C for about 22 minutes.
Cook vanilla custard with milk and sugar. (according to instructions)
Mix in sour cream and vanilla extract in the still warm custard.
Pour custard gush over the cake, spreading evenly.
Bake again for 14 minutes
Remove and leave to cool.
Whip cream (with cream stiffener) and pipe rosettes with a piping bag. Decorate with mint or lemon balm leaves.
Tastes best cold!