Preheat oven to 180°C/fan 160°C/gas mark 5.
Cream together the sugar and margarine in a bowl.
Add the Fate Low Protein All-purpose Mix and baking powder and rub together with your hands until it resembles breadcrumbs.
In a separate bowl, mix together the ProZero, egg replacer and vanilla essence.
Add the ProZero mixture to the dry mixture and mix until well combined.
Separate the mixture evenly into 4 bowls.
Add 3 drops of red food colouring to the first bowl and stir, add more red food colouring until you achieve the desired colour.
Repeat step 7, adding a different colour Dr Oetker Gel Food Colour to each bowl of cake mixture.
Pour a different bowl of mixture into each corner of a lined 8cm cake tin.
Move a knife gently through the mixture to swirl the colours together and bake in the oven for 25 minutes or until a knife can be inserted and comes out clean.
When the cake is cool, mix together the icing sugar and water and pour over the cake.
Serving suggestion: Add a mixture of permitted cake decorations
Fate Low Protein All-Purpose Mix and ProZero are foods for special medical purposes and must be used under medical supervision.