Moravian Sugar Cake

Method

1. Put 70 g of peeled, chunked floury potato in a small pan, cover with unsalted water and simmer 12-15 minutes until it collapses when pressed. Drain over a jug, reserving 50 ml of the starchy cooking water. Mash the potato completely smooth with no lumps and leave both potato and water to cool to lukewarm, about body temperature.
2. In a mixing bowl whisk together 120 g of plain flour, 20 g of caster sugar, 2 g of fast-action yeast and 1 g of fine salt. Add the mashed potato, 50 ml of the lukewarm potato water, 1 tbsp of beaten egg and 8 g of softened butter. Stir to a shaggy dough, then knead on a lightly floured surface for 8 minutes; it should stay soft and a little tacky, so resist adding much extra flour.
3. Shape the dough into a ball, return it to the lightly greased bowl, cover and leave in a warm spot for 1 to 1.5 hours, until doubled and domed and a fingertip pressed in leaves a dent that fills back only slowly.