For the plum cake, beat the sugar and butter together until fluffy. Gradually add the yolks. Add the rum and continue to beat until fluffy.
Mix flour and baking powder and fold into the butter mixture together with the snow.
Line a small baking tray with baking paper and spread the dough on it. Place the plums on the dough and bake for about 35 minutes. Sprinkle the plum cake eventeull with sugar.