Purée the potatoes. Add the cream and place in a bowl over a pan of hot water.
Beat the butter and sugar in a separate bowl. Melt the chocolate and stir it in. Add the potatoes.
Dissolve the bicarbonate of soda in a little water and add to the mixture. Beat in the egg yolks.
Sift together the flour, salt and baking powder. Fold into the mixture alternately with the milk and rum. Whip the egg whites until stiff, stir in a spoonful and then fold in the rest. Divide the mixture between two prepared cake tins and bake for approx.