Cream butter in a large mixing bowl; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition. Stir in vanilla.
Sift together flour, cocoa, soda, and salt in a medium mixing bowl; add to creamed mixture, stirring until well blended. Stir in walnuts. Pour batter into a well-greased 9-inch square baking pan.
Bake at 350° for 20 minutes. Remove from oven; top with marshmallows. Return to oven; bake an additional 5 minutes. Cool completely in pan. Spread with frosting. Cut into 2- x 1- inch bars.