1. Preheat the oven to 180°C (350°F). Grease and flour a rectangular cake pan. To easily remove the cake later, line the pan with a piece of parchment paper, allowing it to overhang. This will help us lift the cake out easily.
2. Grate the zucchini using the fine side of the grater. Place them in a sieve and press out all their liquid with your hands. You should have about 1 and ⅔ cups of grated, drained zucchini.
3. In a bowl, sift together the flour, cocoa powder, baking powder, baking soda, vanilla, and add the salt and ⅔ cup of the coarsely chopped chocolate. Add the two types of sugar, the melted butter, and the sunflower oil to the mixer bowl. Beat for 2-3 minutes, then add the eggs one by one. Add the yogurt and the drained zucchini. Lower the mixer speed and gradually add the flour mixture, spoonful by spoonful.
4. Pour the batter into the prepared pan and sprinkle the remaining chocolate over the top.
5. Bake in the preheated oven for 50 minutes. To check if it's done, insert a wooden skewer into the cake; if it comes out clean, it's ready. Let it cool before turning it out of the pan.
