125g butter
155g Chelsea Caster Sugar
1 large potato, cooked and mashed
60g plain (dark) chocolate, melted
3 Tbsp cocoa powder, sifted
2 eggs
1 Tbsp lemon juice
185g self-raising flour, sifted with 1/2 tsp ground nutmeg
60g walnuts, chopped
3 Tbsp milk

Icing:
155g plain (dark) chocolate, melted
250g thick sour cream

Place butter in a large mixing bowl and beat until light and creamy. Add Chelsea Caster Sugar a little at a time, beating well after each addition. Beat in potato, chocolate, cocoa powder and eggs one at a time. Stir in lemon juice.
Combine flour mixture and walnuts. Fold lightly through butter mixture alternatively with milk. Spoon mixture into a greased and lined 23cm round cake tin. Bake at 180'C (350'F/Gas 4) for 35 - 40 minutes, or until cooked in the centre when tested with a skewer. Stand in tin for 5 minutes before turning out onto a wire rack to cool.
To make icing, place chocolate and sour cream in a mixing bowl and beat until combined. Spread over cold cake.