Peel, wash and cut the sweet potatoes into large pieces, cover and cook in water for about 25 minutes until very soft. Drain, mash finely with a potato masher and pass through a sieve.
Weigh 330 g puree and let it cool down.
Grease a ring cake tin (approx. 2.5 l capacity) and sprinkle with breadcrumbs. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Whisk eggs, sugar, vanilla and 1 teaspoon salt with the whisks of the mixer until creamy.
Fold in sweet potato puree. Mix flour and baking powder. Stir flour mixture and oil into the mixture.
Cut the dough in half. Stir in half of the cocoa, sour cream and milk. Pour the dough alternately into the ring cake mould and pull through spirally with a fork. Bake in a hot oven for 1-1 1⁄4 hours. Remove the cake, let it cool down and turn it over.
Dust with icing sugar.