Possibly derived from potato pudding?
There are a lot of very old potato pudding recipes — which proves that sweetening of potatoes was quite common.
Potato Pudding, Sweet.
Bake half a dozen large potatoes, and when they are done enough break them open and scoop out the contents with a spoon. Beat them lightly, and with a quarter of a pound of the potato flour put three ounces of clarified butter, half a teaspoonful of finely-minced lemon-rind, a dessert-spoonful of lemon juice, a pinch of salt, three table-spoonfuls of powdered sugar, and three table-spoonfuls of milk or cream. Beat the pudding for five or six minutes, then add separately the yolks and well-whisked whites of three eggs. Butter a plain mould, ornament it with dried fruit or slices of candied peel, pour in the pudding, and bake in a well-heated oven, or steam the pudding if preferred. Turn it out before serving, sift sugar thickly over it, and garnish the dish with jam. Time to bake, three-quarters of an hour; to steam, one hour. Probably cost, 1 [cent]. Sufficient for five or six persons."
~Cassell's Dictionary of Cookery, [Cassell, Petter, Galpin & Co.:London] 1875
