For the sheet cake with plums, first separate the eggs. Beat the egg whites until stiff, add a little lemon juice and continue beating. Mix soft butter, powdered sugar and vanilla sugar until creamy. Add egg yolks and rum and mix until fluffy.
Combine flour with baking powder – mix in. Carefully fold in the beaten egg white. Line a baking tray with baking paper and spread the dough on it – top with the seeded doughnut halves.
Bake in the oven at medium heat (approx. 170 °C).