Beat egg yolks with salt, sugar, vanilla sugar, orange and lemon zest until very foamy. Stir in oil and rum. Beat the egg whites and sugar until whipped egg whites. Mix the flour well with the cocoa powder and fold into the yolk mixture with the beaten egg whites, fold in the apple slices. Pour the mixture into a prepared cake tin and bake in a preheated oven at 170 °C for approx. 40 to 50 minutes, then leave to cool. For the lemon glaze mix all ingredients well and spread on the cooled cake.