Make the shortcrust pastry – mix up the flour and sugar, add cold diced butter and chop up with a knife. Add a slightly whisked up egg and press into even pastry (you can also prepare it in a food processor). Place into the fridge for half an hour.
Then roll up the dough and cover an oven tray with it, punch a few holes into it with a fork and bake under 190 °C for 10 minutes or until the pastry starts to turn brown.
For the biscuit, whisk the eggs and sugar into a firm foam (8–10 minutes), mix up the flour and potato starch, sift into the egg foam and stir up gently. Cover the pre-baked pastry with rhubarb pieces (no need to peel young stems), sprinkle with sugar, poor the biscuit on top and spread around evenly. Lower the oven heat to 180 °C and bake the cake for 20–25 minutes. Sprinkle with powdered sugar before serving.