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1
Position a rack in the center of the oven and preheat to 325u00b0F. Coat a 9-by-13-by 2-inch cake pan or dish with nonstick cooking spray. If you're using a metal pan, line the bottom and sides with foil or parchment paper and spray again.
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2
Set a box grater on some paper towels and grate the zucchini on the large holes. Spread it out on the paper towels, top with more paper towels, and press gently to remove excess moisture.
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3
Sift the flour, cocoa powder, cinnamon, baking soda, baking powder, and salt into a large bowl. Whisk the sugar, buttermilk, oil, eggs, and vanilla in a medium bowl until very smooth. Make a well in the dry ingredients and pour in the wet ingredients. Whisk, gradually drawing in the dry ingredients, just until smooth. Fold in the zucchini and chocolate chips with a silicone spatula until evenly incorporated. Spread the batter in an even layer in the prepared pan.
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4
Bake the cake until a toothpick inserted in the center comes out clean and the top springs back a little when lightly pressed with a fingertip, 45 to 50 minutes. Cool completely in the pan on a rack.
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5
Meanwhile, make the frosting: Bring the sweet potato puree to a simmer in a large saucepan over medium heat, stirring frequently. Remove from the heat and add the chocolate. Stir until smooth. Cool, stirring occasionally, until the mixture is at room temperature and the consistency of canned frosting. It should hold soft peaks when you lift the spatula from the pan but not be stiff. Spread the frosting all over the top of the cake, creating swoops and swirls.
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6
The cake is best the day it's made, when the chocolate chips are still a bit melty and the frosting is soft, but it will keep in the refrigerator for up to 3 days.
