Butter and spread with cocoa powder a cake pan. Set aside. Keep in mind that the cake will be baked at 180* C. But I do not suggest to turn the oven on at this point, we still have time.
Then, start with preparing the ingredients. So, remove butter and eggs out of the refrigerator and then melt bittersweet chocolate. Cut the chocolate into small pieces and add it at a bain marie. Once it has melted (you can stir with a wooden spoon in order to help chocolate to melt), set it aside in order to come at room temperature.
Continue with grating zucchini. Remember that we need the moisture of the zucchini so we do not squeeze it. Leave aside.Now, you can turn the oven on at 180* C.
Like most cakes, put all the dry ingredients in a bowl. So, after sifting the flour (well if you do not sift it, it's not the end of the world) combine it along with cocoa, soda and salt. Leave aside.
Then, in a stand mixer, at medium speed, cream together the butter with the two types of sugar, for about five minutes. Then add the oil, one egg at a time and finally the vanilla extract.
Add the flour mixture alternately with the yogurt in 3 additions each.
Remove the bowl from the mixer and add the grated zucchini and then the melted chocolate (check if it has come to room temperature before adding it to the mixture.)
Pour the mixture into the cake pan and bake for about 45 minutes. As each oven is different, it is good to do the toothpick / knife test before removing it out of the oven. The difference here with the majority of the cakes is that we do not want to come out clean but with some moist crumb. Let it cool.