Prepare the shortcrust pastry: Combine flour, cold butter, sugar, and salt in a mixing bowl. Cut the butter in until the mixture resembles coarse crumbs. Add the egg yolk and mix briefly until a cohesive dough forms. Wrap the dough and chill in the refrigerator for at least 30 minutes.

Preheat oven to 180°C (350°F). Grease and line a 26 cm springform pan. On a lightly floured surface, roll out the chilled dough and press it evenly into the bottom and slightly up the sides of the prepared pan.

Prepare the rhubarb: Toss the cut rhubarb pieces with 50g of sugar and 1 tablespoon of cornstarch. Spread the rhubarb mixture evenly over the pastry base.

Prepare the custard: In a medium bowl, whisk together the milk, heavy cream, eggs, 100g sugar, vanilla extract, and 1 tablespoon of cornstarch until smooth and homogenous. Do not over-whisk.

Pour the liquid custard mixture carefully over the rhubarb layer in the pan. Ensure the custard covers most of the rhubarb pieces.

Bake for 45 to 55 minutes, or until the custard is set (it should only have a slight wobble in the very center) and the edges are golden brown. If the edges brown too quickly, loosely cover the pan with foil.

Allow the Kuchen to cool completely in the pan on a wire rack before carefully removing the springform ring. Serve chilled or at room temperature, often dusted with powdered sugar.