Preheat the oven to 350F.
In a large bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda and salt. In a smaller bowl, whisk together the yogurt, oil, eggs and vanilla. Stir into the dry ingredients, along with the zucchini, stirring just until the two are incorporated and there are no more streaks of flour. Stir in the chocolate chips, if you're using them.
Scrape the batter into a parchment-lined (or buttered) 9x13-inch baking pan and bake for 30 minutes, or until the cake is springy to the touch. You can cool and frost it if you like, but I like keeping it plain and snacky. The batter would also make great cupcakes/muffins - bake in paper-lined tins for 15-20 minutes, or until springy. Makes 1 cake; serves 12 or so.