Preheat the oven to 350 degrees. Start by grating the zucchini, then spread it on paper towels to absorb some moisture.
In a large bowl, mix oil, sugar, brown sugar, eggs, and vanilla until creamy. Stir in the grated zucchini and sour cream.
Add cocoa powder, flour, baking soda, baking powder, and salt—mix only until combined to avoid over-mixing. Fold in chocolate chips.
Pour the batter into a 9x13 pan (lightly sprayed with non-stick cooking spray). Bake for 24-28 minutes until a toothpick comes out clean from the center.
Cool completely and then frost with the homemade chocolate buttercream frosting.