Preheat the oven to 350 degrees F. Butter a 9-by-4-by-3-inch loaf pan and dust with flour, tapping out the excess.
Whisk the flour, cocoa, baking soda and salt together in a medium bowl.
Beat the butter and sugar in a medium bowl with an electric mixer on medium-high speed until light and fluffy, about 4 minutes. Continue to mix and drizzle in the oil and beat until incorporated. Beat in the egg and then the vanilla.
Reduce the mixer speed to low and beat in the flour mixture in 3 additions, alternating with the buttermilk in 2 additions, beginning and ending with the flour, scraping the sides of the bowl occasionally. Fold the zucchini and chocolate into the batter and pour into the prepared pan. Center the zucchini slice, lengthwise, on top of the batter.
Bake until a toothpick inserted in the center comes out clean, about 55 minutes. Cool completely in the pan on a wire rack.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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