Line the baking pan with baking paper.
Slowly melt dark chocolate together with butter and let it cool down a bit.
Rasp zucchini coarsely.
Mix eggs, sugar, vanilla and salt until homogeneous.
Add vegetable oil and mix again until homogeneous.
Mix flour, cocoa powder and baking soda.
Add the dry flour mixture to the liquid vegetable oil-egg-sugar mixture and mix just briefly to form a viscous batter. If your batter is too runny, add a little more flour.
Add grated zucchini to the batter and mix again briefly until homogeneous.
Add the liquid chocolate-butter mixture and mix it again briefly to a homogeneous thick batter.
Pour the batter into the baking pan, smooth it out and bake the chocolate cake in a preheated oven at 180 °C top and bottom heat for about 20 minutes.
Let the chocolate cake cool to warm in the baking pan, then remove it very carefully from the baking pan with the help of the overhanging baking paper and let it cool completely.