Separate the egg. Cut butter into pieces. Mix butter, 75 g sugar, flour and egg yolk first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Wrap the dough in foil and chill for about 30 minutes. Wash and clean the rhubarb and cut into very thin slices.
Roll out the dough round on a floured work surface (approx. 30 cm Ø). Grease tart tin with lift-off base and line with the dough. Whip the protruding edges inwards and press down firmly. Prick the base several times with a fork. Whip 3 egg whites until stiff, adding vanillin sugar and 150 g sugar. Continue beating until the sugar has dissolved. Mix rhubarb and starch well in a bowl. Fold the rhubarb into the beaten egg white and spread loosely in the tart. Bake the tart on the lower shelf in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas mark 3) for 30-35 minutes. Cover the tart with aluminium foil or a baking tray if the surface becomes too dark.
Continue beating until the sugar has dissolved. Mix rhubarb and starch well in a bowl. Fold the rhubarb into the beaten egg white and spread loosely in the tart. Bake the tart on the lower shelf in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas mark 3) for 30-35 minutes. Cover the tart with aluminium foil or a baking tray if the surface becomes too dark. Remove the tart and let it cool down on a chick rack. Dust with icing sugar before serving
waiting time approx. 1 1/2 hours