Wash, clean and finely grate the zucchini. Wash the lemon balm and remove the leaves. Finely chop lemon balm and pistachios in the universal chopper.
Cream butter, sugar, vanilla sugar and 1 pinch of salt with the whisk of the mixer. Stir in the eggs one by one. Mix flour and baking powder. Add the pistachio mixture, zucchini and the zest of 1 lime. Spread the dough into a greased, flour-dusted ring cake tin (2.5 l capacity).
Bake the ring cake in a preheated oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer) for 45-60 minutes. Leave to cool in the mould for approx. 15 minutes, then carefully turn out onto a rack. Let it cool down.
For the icing, mix icing sugar, peel of 1 lime and lime juice to a smooth icing. Spread the icing on the cake. Sprinkle with gold pearls and zest of 1 lime. Leave to dry.