In a bowl, combine flour, cocoa, baking soda and salt. Combine zucchini, sugar and oil. Add to dry ingredients. Mix well. Stir in walnuts and vanilla. Pour into a greased 13x9" baking pan. Bake at 350 degrees for 35-40 min. or until a toothpick comes out clean. Cool on a wire rack for 30 min.
In a saucepan, melt butter. Stir in sugar and milk. Bring to a boil over med. heat, stirring frequently. Boil and stir 1 min. Remove from heat. Stir in chips and marshmallows until melted. Add vanilla. Spread over brownies. sprinkle with walnuts if desired.