Preheat oven to 350 degrees F (175 degrees C).
In large bowl, whisk together flour, cocoa powder, baking soda, cinnamon, and salt.
In another large bowl, stir together sugar and egg and egg yolk until smooth, 1 minute.
Add melted butter and vanilla extract and mix until smooth.
Add zucchini, then add flour mixture in 3 additions.
Fold in chocolate chips.
Bake in preheated oven for 60 to 70 minutes, or until a toothpick inserted into the center of a loaf comes out clean.
Let the loaf cool on a rack for about 10 minutes before turning out onto a rack to cool completely.