Grease a ring cake tin (approx. 2 1⁄2 l capacity) and dust with flour. Separate the eggs. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Wash, clean and grate zucchini and apple.
Cream 200 g butter, sugar, vanilla sugar and 1 pinch of salt with the whisks of the mixer. Stir in the egg yolks one after the other. Mix 300 g flour, baking powder and cocoa and stir in. Add zucchini and apple.
Beat the egg white until stiff and fold in carefully.
Fill the dough into the mould and smooth it down. Bake in a hot oven for about 1 1⁄4 hours (chopstick test!). If necessary, cover the cake with aluminium foil about 20 minutes before the end of the baking time. Take out and let it cool in the tin for about 30 minutes.
Drop onto a cake rack and let it cool down.
For the icing, mix icing sugar, lemon juice and 1-2 tbsp. water to a thick icing. Spread on the cake and let it dry. Sprinkle with mint sugar as desired.