Turn on the oven at 365°F. Cut the zucchini into chunks and blend with the apple juice until smooth. Set aside.
Combine whole wheat flour, brown sugar and mascobado in a large bowl, add sifted cocoa, starch and baking powder. Pour the zucchini apple smoothie and mix using a whisk to obtain a smooth cake batter. Add bycarbonate and whisk a little more to distribute it evenly.
Pour the batter into a 9 or 10 inch cake pan, you don't need to oil it if using silicone, otherwise line the pan with parchment paper.
Bake for 30 minutes and test the cake with a toothpick, it has to come out dry. If needed, bake the courgette chocolate cake 5 minutes more, remove it from the oven and let it cool down.
If you want, drizzle the courgette chocolate cake with melted dark chocolate then serve.
Note
The courgette chocolate cake keeps moist at room temperature for a couple of days, closed on a glass cake stand or in an airtight container, and up to 6 days in the fridge.