Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×13 inch baking pan.
In a large bowl, mix together the oil, sugar, and 2 teaspoons vanilla until well blended. Combine the flour, 1/2 cup cocoa, baking soda, and salt; stir into the sugar mixture. Fold in the zucchini and walnuts. Spread evenly into the prepared pan.
Bake for 25 to 30 minutes in the preheated oven, until brownies spring back when gently touched.
Make frosting while brownies cool. Melt together the 6 tablespoons of cocoa and margarine; set aside to cool.
Meanwhile, blend together the confectioners' sugar, milk, and 1/2 teaspoon vanilla. Stir in the cocoa mixture. Spread over cooled brownies before cutting into squares.