Combine the sour cream and ¼ cup icing sugar, cover and refrigerate.
Put the pitted prunes, cinnamon and cloves into a small bowl. Top with hot water, mix well, and set aside.
Put the rinsed buckwheat into a large saucepan along with the milk and salt. Bring to the boil over medium heat, stir occasionally, simmer for 10-15 minutes and watch that the milk doesn’t boil over.
Turn the heat off and let it sit until room temperature. If there’s excess milk when the buckwheat has cooled, drain it off.
Preheat the oven to 200°C. Grease and line a 20 cm springform cake tin.
Drain the soaked prunes really well, discard the cloves and squish the prunes with your hand to break them up into a rough paste. Mix this through the cooled buckwheat, followed by the sugar, cream, vanilla, melted butter and whisked eggs.
Pour the batter into the prepared baking tin, smooth the surface and arrange the quartered plums over the top.
Bake in the oven for 1-1½ hours, or until it’s cooked all the way through. Cool completely before removing from the cake tin.