Half-boil some fine sweet potatoes; peel them; and when cold, grate as much as will weigh half a pound. If boiled long enough to become soft, they will render the cake heavy. Stir together in a deep pan, half a pound of fresh butter, and half a pound of powdered loaf-sugar, till quite light and creamy. Then add a tea-spoonful of powdered mace, nutmeg, and cinnamon, all mixed together; and the juice and grated rind of two large lemons or oranges. Beat in a shallow pan six eggs till very smooth and thick; and stir them into the pan of butter and sugar in turn with the grated sweet-potatoe, a little of each at a time. Then stir the whole very hard. Butter a deep tin pan with straight sides. Put in the mixture, and bake it well. If you want more cake than the above quantity, double the proportions of each ingredient; but bake the mixture in two pans, rather than in one. Ice it when cold, adding a little lemon or orange-juice to the icing. In spreading the icing, begin by heaping it on the centre of the cake, and then gradually bringing it all over the top and sides, dipping the knife, frequently, into a bowl of cold water.