Wash zucchini, grate finely and drain on a sieve. Melt the butter and 100 g chocolate and allow to cool. Roughly chop the rest of the chocolate.
Mix all dry ingredients together.
Beat the sugar and eggs until frothy. Add butter-chocolate mixture and yoghurt and stir well. Then add all the dry ingredients and stir only briefly so that everything is well mixed.
Briefly stir in the zucchini and chopped chocolate and pour the batter into a greased and floured square springform pan. Put in the preheated oven (175 degrees top / bottom heat) and bake for 35 - 40 minutes.