In a medium bowl, combine the flour, sifted cocoa powder, baking soda, and salt. Whisk together and set aside.

In a large bowl, combine the brown sugar, oil, melted butter, buttermilk, eggs, and vanilla extract. Whisk until combined.

Stir in the dry ingredients until barley combined, it is ok if there is still some flour not mixed in. Gently squeeze out some of the moisture in the zucchini. You want the zucchini to still have some moisture, but not full of liquid. Add the zucchini and chocolate chips and stir until just combined. Don't over mix.

Pour the cake batter in the prepared pan and smooth with a spatula. Bake for 25-32 minutes or until a toothpick comes out clean when inserted into the center or the cake.

Let the cake cool completely. While the cake is cooling, make the chocolate frosting. In the bowl of a stand mixer or with a hand mixer, beat the butter until smooth, about 1 minute. In a large bowl, sift together the confectioner's sugar and the cocoa powder. Add the mixture to the butter and beat, slowly adding in the milk. Add the vanilla extract and salt. Stop and scrape down the sides. Beat until frosting is smooth and fluffy.

Frost the cooled chocolate zucchini cake with chocolate frosting. Cut the cake into squares and serve.