Wash the potatoes and cook for about 20 minutes. Quench, peel and let cool down.
Mash potatoes or grate them finely. Knead under the stollen dough. Grease a springform pan (26 cm Ø). Pour in the dough and press flat with your hands. Melt butter. Spread the cake with it and sprinkle with sugar.
Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for 20-25 minutes. Let them cool down.