Clean and wash the rhubarb, drain well and cut into even pieces. Mix with 50 g sugar and set aside. Roughly chop the chocolate. Mix fat and 200 g sugar until creamy. Add lemon zest, stir in eggs one after the other. Mix flour, cornstarch and baking powder and stir briefly into the mixture. Fold in chocolate.
Spread the dough evenly on a greased and floured baking tray (32 x 39 cm) or the fat pan of the oven. Spread rhubarb in strips on top. Bake in the preheated oven (electric cooker: 200°C/ convection oven: 175 °C/ gas: level 3) for approx. 25 minutes. In the meantime, beat the egg whites until stiff, add 50 g sugar and finally stir in the lemon juice. Spread the meringue mixture with a spoon as tuffs on the cake. Bake the cake for another 5 minutes at the same temperature until the meringue tuffs are lightly browned