For the base: Mix flour, sugar, and salt in a bowl. Cut in cold butter until crumbly. Mix egg yolk and cold water, add to flour mixture, and knead briefly until a dough forms. Wrap in plastic and chill for 30 minutes.

Preheat oven to 180°C (350°F). Roll out the dough and line a 24-26 cm (9-10 inch) springform pan. Prick the base with a fork.

Blind bake the base for 15 minutes. Remove parchment paper and weights, then bake for another 5 minutes until lightly golden. Let cool.

For the rhubarb topping: Toss chopped rhubarb with sugar and vanilla extract. Spread evenly over the cooled cake base. Bake for 20-25 minutes until rhubarb is tender.

For the cream filling: Soak gelatin sheets in cold water for 5 minutes. Heat heavy cream and sour cream gently (do not boil). Squeeze out excess water from gelatin and dissolve it in the warm cream mixture. Stir in powdered sugar and vanilla extract.

Let the cream mixture cool slightly. Pour it carefully over the rhubarb layer. Chill the cake in the refrigerator for at least 4 hours, or until the cream is set.

To serve, carefully release the springform pan. Slice and enjoy this refreshing Rhabarber Sahne Torte!