For the plum cake from the tray, separate the eggs and beat the yolks together with the sugar and the liquid butter until well creamy.
Add the sour cream and stir it in. Sift the flour with the baking powder and gradually stir into the batter together with the rum.
Beat the egg whites with a pinch of salt until stiff and then carefully fold into the dough.
Spread the dough on a baking tray (30 x 40 cm) lined with baking paper and cover densely with the plums. Sprinkle with cinnamon and brown sugar and bake the plum cake from the tray in the preheated oven at 180 °C for 35-40 minutes.