Pre-heat the oven to 350 degrees. Grease and flour a 9×13 inch baking dish. In a large bowl mix together oil, sugar and 2 teaspoons of vanilla extract until well blended. In another bowl combine the flour, ½ cup cocoa powder, baking soda, and salt. A little at a time stir into the sugar mixture. Fold in zucchini and walnuts. Spread the mixture evenly into the prepared baking dish.
Place into the oven and bake for 25-30 minutes until the brownies spring back when gently touched. Remove from the oven and allow them to cool.
To make the frosting- In a small saucepan over low heat melt together the cocoa powder and margarine. Set aside to cool. In a medium bowl blend confectioner’s sugar, milk, and ½ teaspoon vanilla extract. Stir in the margarine/cocoa mixture. Spread over cooled brownies and cut into squares.