Wash and clean the rhubarb and cut into small pieces. Cut the fat into pieces. Drain the cranberries in a small sieve. Cream fat, sugar and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one by one.
Mix flour and baking powder and stir in alternately with 3 tablespoons of rhubarb juice. Finally fold in the rhubarb pieces. Pour the dough by the spoonful into a greased and floured box form (1 1/2 litre capacity), sprinkle small cranberry spots on the dough several times. Bake the cake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 50-60 minutes. If the surface becomes too dark, cover with aluminium foil. Remove the cake from the oven, let it cool down for about 15 minutes, then carefully remove it from the tin and let it cool down on a cake rack. Sieve the icing sugar into a bowl. Add egg white and 1-2 tablespoons rhubarb juice and mix to a thick icing. Colour the icing pink with food colouring.
If the surface becomes too dark, cover with aluminium foil. Remove the cake from the oven, let it cool down for about 15 minutes, then carefully remove it from the tin and let it cool down on a cake rack. Sieve the icing sugar into a bowl. Add egg white and 1-2 tablespoons rhubarb juice and mix to a thick icing. Colour the icing pink with food colouring. Cover cold cake with the icing and let it dry for about 1 hour, then cut into slices