Combine the flour, sugar, cocoa powder, baking powder, salt, and dark chocolate chips in a large mixing bowl.

In a separate bowl, beat the eggs lightly. Add the vegetable oil, vanilla extract, sour cream, and grated zucchini to the eggs and mix well.

Add the wet ingredients to the dry ingredients and mix until just combined. The batter will be dry initially.

Let the batter sit for 5 minutes, allowing the zucchini to release moisture. After 5 minutes, mix the batter again; it should now be wetter due to the released liquid from the zucchini.

Pour the batter into the prepared 9x9-inch baking pan and spread it evenly.

Bake in the preheated oven for 35-45 minutes or until a toothpick inserted into the center comes out with moist crumbs (not wet batter). Remove from the oven and let it cool completely in the pan.

While the brownies are cooling, prepare the chocolate frosting. In a medium bowl, whisk together the melted butter, cocoa powder, milk, powdered sugar, vanilla extract, and a pinch of salt until smooth and creamy. Adjust the consistency by adding more milk or powdered sugar as needed.

Once the brownies are cooled, spread the chocolate frosting evenly over the top.

Allow the frosting to set before cutting the brownies into squares and serving.