Preheat oven to 350 degrees. Grease and flour 9 x 13 pan
In large bowl, mix together the oil, sugar, and 2 tsp. vanilla extract. Combine the flower, 1/2 c. cocoa, baking soda, and salt; stir into the sugar mixture. Fold in the zucchini and walnuts. Spread in pan.
Bake for 25 to 30 minutes until brownies spring back when touched.
To make the frosting, melt together the cocoa and butter; set aside to cool. In a medium bowl, blend together the powdered sugar, milk, and vanilla. Stir in the cocoa mixture. Spread over cooled brownies.