Clean, wash and cut the rhubarb into pieces of about 2 cm. Mix a tablespoon of flour into the rhubarb. Cream the soft fat, sugar and vanilla sugar with the whisk of the hand mixer. Stir in 4 eggs one after the other. Mix 250 g flour, coconut flakes and baking powder and stir into the fat-egg mixture alternately with the milk. Fold in rhubarb pieces. Fill the dough into a greased and breadcrumbed box form (1.5 litres capacity) and smooth it down.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 60 minutes. Let cool in the mould for about 10 minutes. Turn out onto a cake rack and allow to cool completely. Mix icing sugar, 1-2 tablespoons of water, egg white and a few drops of food colouring with the whisk of the hand mixer until smooth. Cover the cake with the icing and sprinkle with coconut chips. Leave to dry