For the plum cake, first separate the eggs. Beat the egg whites with the granulated sugar until stiff. Beat egg yolks, powdered sugar and softened butter until fluffy. Mix in the rum.
Mix flour with baking soda and stir in. Carefully fold in the beaten egg whites. Pour into a greased and floured cake pan and bake briefly in the oven at approx. 170 °C.
In the meantime, remove the seeds from the plums, cut them in half and spread them on the cake – finish baking.