Wash the potatoes and cook in boiling water for about 20 minutes. Grease a springform pan (26 cm Ø). Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Drain potatoes. Peel hot, mash to puree and let cool down a bit.
Chop the hazelnuts. Cream 100 g butter and 200 g sugar with the whisks of the mixer. Stir in the eggs one after the other. Stir warm mashed potatoes, 100 g nuts and baking powder into the egg mixture.
Pour into the mould. Pre-bake in a hot oven for about 10 minutes.
In the meantime, wash the plums, cut them in half and stone them. Spread on the cake. Sprinkle with remaining nuts and 4 tablespoons of sugar. Bake for another 40 minutes. Remove the cake and let it cool down for about 10 minutes.
Carefully remove from the mould and leave to cool on a cake rack.