Preheat your oven to 350°F. Line a 9x13" baking pan with foil and spray with cooking spray. Set aside.

In a medium bowl, whisk together the flour, cocoa, baking soda, and salt. Set aside.

Using an electric mixer fitted with a paddle attachment, mix the oil, sugar, and vanilla until well combined. Add the dry ingredients and stir. Fold in the shredded zucchini. Let the mixture sit for a few minutes so the batter can absorb the moisture from the zucchini.

If the mixture is still powdery, add up to 5 tablespoons of water, one tablespoon at a time, stirring well after each addition. The batter should be very thick but not powdery. (The amount of water depends on how wet your zucchini is.) If using, add the chopped walnuts. You might need to use your hands to work the water in instead of a spoon. The dough will be thick like cookie dough. Be careful not to add too much water. Spread the batter evenly in the prepared pan.

Bake for 25-30 minutes until the brownies spring back when gently touched.

For the frosting, whisk together the melted butter, cocoa, salt, and powdered sugar. Whisk in the milk and vanilla extract until smooth. Spread the frosting over the cooled brownies. Cut into squares and chill to set slightly. The frosting will harden slightly on top but stay wet and gooey underneath.

Store the brownies in an airtight container and eat within 2 days. Chilling them can make them last an extra day. These are "fork" brownies because they are so gooey.