Mix flour, cocoa powder, brown sugar, cornstarch, instant coffee powder, baking powder and salt.
Mash banana with a fork, add plant milk, vanilla as well as vegetable oil and mix it. Take a little less vegetable milk first and add the rest as needed in the next step.
Add the liquid banana mixture to the dry flour mixture and mix it into a creamy dough. The batter should not be too runny, but not too firm either. If it's too firm for you, add more plant milk.
Coarsely grate the zucchini. Stir the shredded zucchini and melted dark chocolate into the banana flour mixture.
Line the baking pan with baking paper, pour the creamy batter into the pan, sprinkle with chopped dark chocolate and bake the chocolate zucchini bread in a preheated oven at 170 °C for about 1 hour. Let it cool down a bit in the baking pan, then take it out together with baking paper and let it cool down completely.