Cream margarine and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Sift cocoa and flour together, beat in gradually. Stir in walnuts. Spoon batter into a greased 9 x13 inch pan. Bake at 350F oven for 30 - 35 minutes. Immediately sprinkle top of cake with marshmallows. Return to oven for 3 - 5 minutes until marshmallows are slightly puffed. Cool 30 minutes. For icing, cream margarine. Combine icing sugar and cocoa, and beat in alternately with milk. Beat until icing is light and fluffy. Spread over marshmallow layer. Chill l to serve - cut in bars.
