Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease a springform pan (20 cm Ø). Separate eggs for the dough. Beat the egg white until stiff. Wash orange and lemon hot, dab dry.
First grate both peels finely, then squeeze the fruit (yields approx. 100 ml juice). Cream 200 g butter, sugar and citrus peels with the whisks of the mixer. Stir in the egg yolks one after the other.
Briefly stir in flour, buttermilk and 4 tbsp. squeezed citrus juice. Fold in the beaten egg white. Smooth the dough in the form. Bake in a hot oven for about 45 minutes.
Take out the cake. Immediately prick several times with a wooden skewer. Sprinkle with 4 tbsp. citrus juice. Let it cool down.
For the icing, mix icing sugar with approx. 2 tbsp. citrus juice to a smooth icing. Remove the cake from the mould. Pour the icing over the cake and let it dry. Decorate with citrus slices in syrup to serve.
Tip: Make your own citrus slices in syrup! For this, wash 1/2 organic lemon and 1/2 organic orange hot, dab dry and cut into thin slices. Heat 100 ml water and 175 g sugar in a wide pan over low heat until the sugar has dissolved. Add orange slices, bring to the boil briefly and simmer for about 15 minutes, turning from time to time. Remove from the stock and drain on a cake rack, making sure to collect the syrup! It is great for mixing drinks, for homemade lemonade or orange sauces.