Preheat oven to 350°. Butter a 9"-x-5" loaf pan and dust with cocoa powder, tapping out any excess.

In a large bowl, whisk together flour, cocoa powder, baking soda, cinnamon, and salt.

In another large bowl, stir together sugar, egg, and egg yolk until smooth, 1 minute. Add melted butter and vanilla and mix until smooth, then fold in zucchini. Gradually add flour mixture, then fold in chocolate chips. Transfer batter to prepared pan.

Bake until a toothpick inserted into the center comes out clean, about 50 minutes.

Let bread cool in pan for 10 minutes, then invert onto a wire rack to cool completely.

Sprinkle with flaky sea salt, slice, and serve.