Cream the butter and sugar together until light in colour and creamy. Beat in each egg separately until the mixture is smooth again.
Sieve together the flour, cocoa and baking powder and stir these in lightly to the butter and sugar mixture. Lastly stir in the potato and milk.
Pour the mixture into a greased and lined 10 inch cake tin then bake in a moderate oven 190C / 170C fan / gas mark 5 for 35-40 about minutes or until a skewer comes out from the centre cleanly.
Once cooked turn out onto a cooling rack and leave to cool.
For the icing, zest the orange and set the zest to one side. Sieve the icing sugar into a large bowl then whisk in the juice of half an orange to make a thick paste that will pour over the cake but not completely run off.
Pour the icing over the cooled cake then sprinkle over the orange zest.
85g mashed Smooth potatoes (such as Desiree)
150g self-raising flour
20g cocoa powder
170g caster sugar
175g unsalted butter, softened
1 rounded tsp baking powder
2 eggs, beaten
3 tbsp semi-skimmed milk
For the icing
150g icing sugar