Preheat oven to 350 degrees F. Butter and dust a loaf pan with cocoa powder.
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.
Transfer batter to prepared dish and bake 50 minutes.
Let cool slightly in pan, then transfer to a cooling rack. Sprinkle with flaky sea salt and serve.