In a large bowl, combine the flour, cocoa, baking soda and salt. In a small bowl, combine the zucchini, sugar and oil; stir into dry ingredients until blended. Stir in walnuts and vanilla.
Pour into a greased 13-in. x 9-in. baking pan. Bake at 350° for 35-40 minutes or until a toothpick comes out clean. Cool on a wire rack for 30 minutes.
In a large saucepan, melt butter; stir in sugar and milk. Bring to a boil over medium heat, stirring frequently. Cook and stir 1 minute or until smooth. Remove from the heat. Stir in chips and marshmallows until melted and smooth; add vanilla. Spread over brownies. Sprinkle with walnuts if desired.