Melt margarine and cocoa. Stir in sugar. Add eggs, flour, vanilla, and nuts. Pour into greased 9 x 13 pan. Bake at 350 for 30 minutes. Cover with marshmallows - place back in oven until marshmallows begin to melt. Remove and let cool for one hour.
Topping - Combine all ingredients - start with margarine and cocoa and vanilla; then add the sugar, adding milk until you reach spreading consistency. Pour over brownies. Cut into squares. Eat!