Preheat oven to 350F
Grease a 8” x 8” casserole dish for thick brownies, or a 9” x 16” dish for thinner brownies. Set aside.
In a large bowl, whisk together the flour, dark cocoa powder, baking soda and salt.
Make a well in the centre of the dry ingredients and add in the white sugar and oil. Beat well to incorporate, then add the vanilla extract, zucchini and milk.
Beat until well-combined and no dry patches remain.
Stir in the chocolate chips to personal preference.
Bake for 25-30 minutes for a 9x16 pan or 35-45 min for a 8x8 pan, until an inserted toothpick comes out clean.
Allow the brownies to cool completely before preparing the frosting.
In a medium-sized saucepan, combine the sugar, dark cocoa powder, butter and milk. Place over medium heat and bring to a boil for 2-3 minutes, stirring constantly.
Remove from heat and stir in the vanilla extract. Let stand for 1 minute, then beat with an electric mixer for 3 minutes.
Pour the frosting over the cooled brownies and smooth with an offset spatula or the back of a spoon.
Let frosting harden before cutting and serving or storing brownies.