Preheat the oven to 170 degrees Celsius/ 340 degrees Fahrenheit. Line a brownie pan (mine is 22 x 24 cm/ 8.6 x 9.5 inches) with baking paper.
Grate the zucchini and roughly chop the cashews.
Place the butter and the roughly chopped chocolate into a small pan and heat gently until melted. Do not let boil. Leave to cool for a few minutes while preparing the rest.
Place the eggs, sugar and vanilla sugar into a bowl and whisk up with a hand-held mixer until frothy.
Mix the flour, cocoa and baking powder together and give over the egg mixture. Incorporate and add the butter-chocolate mixture. Add the zucchini and the cashews and incorporate everything. You can add some chocolate chips here if you like.
Pour this mixture into the prepared pan and bake for 30 to 35 minutes, depending on your oven and also the size of the pan. Insert a toothpick in the middle of the cake. It should come out still a little moist. Leave to cool and dust with icing sugar if desired.
They keep well in an airtight container for several days.