Peel the rhubarb and cut into small pieces. Marinate with sugar, amaretto and vanilla sugar.
For the cake batter, beat the egg whites until stiff. Beat yolks with sugar, salt, lemon juice, marzipan and butter until foamy. Sift flour, cornstarch and baking powder and stir into the yolk-butter mixture. Finally, fold in the beaten egg whites and the marinated rhubarb pieces.
Spread the cake mixture on a baking tray lined with baking paper and bake in the oven on the middle shelf at approx. 180 °C for 20-25 minutes.
In the meantime, for the meringue topping, beat the egg whites with sugar until stiff. Spread on the cake and bake for about 15 minutes until the meringue has a nice color.