For the rhubarb fool: put the rhubarb in a pan with 4 tbsp sugar and 2 tbsp water. Cook over a low heat for 10-15 minutes until tender. Stir in the remaining sugar and leave to cool completely.
Whisk the cream into soft peaks. Strain the juice from the rhubarb into the cream and whisk until blended and thick. Gently stir in the rhubarb to give a marbled effect. Chill for 2 hours.
For the ginger cake: heat the oven to 180C,350F, gas 4. Grease and line a 30 cm x 22 cm|12" x 9" tin with non-stick baking paper.
Put all the cake ingredients into a mixing bowl and whisk or beat well for 2 minutes until well blended. Turn into the tin and smooth the top with a damp palette knife until completely even. Bake for 30-35 minutes until springy to the touch.
Leave to cool in the tin and then turn out. Remove the lining paper and cool on a wire rack.
Cut the cake into squares. Place a spoonful of rhubarb fool on half the squares and top with another square of ginger cake.
For the decoration: put the icing sugar into a small plastic bag or screwtop jar with a little