Preheat oven to 350 degrees. Spray the bottom of a 9x13 inch baking pan with cooking spray.
In a medium bowl, stir together the flour, sugar, cocoa, baking soda, baking powder, salt and cinnamon.
Add the eggs and oil, mix well. Fold in the nuts and zucchini until they are evenly distributed.
Pour into prepared pan. Bake 50-60 minutes, until a knife inserted into the center comes out clean. Cool cake completely before frosting.
For the frosting: combine the brown sugar, butter and milk in a saucepan. Bring to a boil over medium heat; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in vanilla. Cool until lukewarm. Gradually beat in confectioners' sugar until frosting reaches spreading consistency. Frost cake.