This springy, rich chocolate cake has zucchini hidden inside. The zucchini is undetectable but keeps the cake incredibly moist!
Shred or grate your zucchini if you haven't already. I leave the skins on mine. Both finely and coarsely grated zucchini work fine; just don't puree it.
With an electric or stand mixer, combine butter, oil, sugar, vanilla, baking soda, baking powder, and salt, mixing until smooth.(The sugar will begin to dissolve.) Add eggs.
Sift together your flour and cocoa powder if you're worried about lumps. Add half of the flour and cocoa, then the yogurt, and then the rest of the flour and cocoa, mixing until just combined. Stir in shredded zucchini and 1/2 cup of the chocolate chips.
Pour into a greased 9x13-inch pan. Bake at 325F for 30-35 minutes before testing the cake for doneness. (See instructions below, and notes.)
Option 1, for a denser cake: Bake until the top of the cake springs back when touched. The center will be set, and a toothpick inserted into the center will have some dense crumbs (like with brownies). Top the cake immediately with 1 cup of chocolate chips. Return the cake to the oven for at least 5 minutes, and bake until a toothpick inserted into the center comes out clean.
Option 2, for a more traditional cake: Bake until a toothpick inserted into the cake comes out clean (or nearly clean, with just a few crumbs). Top immediately with 1 cup chocolate chips, then return the cake to the oven for 1 minute to soften the chocolate chips.
Remove cake from oven. Immediately smooth out hot chocolate chips into a glaze. Cool at least 10 minutes before serving. You can store it covered at room temperature for up to 3 days, but storing in the refrigerator is recommended.
The baking time for this cake will vary depending on the amount of moisture in your zucchini. Last time I baked it, I used a larger, late-season zucchini, and it took 42 minutes for a toothpick to come out almost clean (for a traditional cake, Option 2) before adding the chocolate chips. If you add the chocolate chips too soon, it may take up to 50 minutes to finish the cake.
Hack together sour milk to substitute for the yogurt by adding 1 1/2 teaspoons vinegar to just under 1/2 cup of milk.
I believe you could swap a third of the all-purpose flour for whole wheat flour without noticing a difference in taste or texture.
This recipe was adapted from King Arthur Flour.