Cream butter and sugar until light and fluffy; beat in cocoa. Add eggs, one at a time, beating well after each addition. Add flour, nuts and vanilla; mix well. Pour batter into greased and floured 9x13x2-inch baking pan. Bake chocolate marshmallow cake at 325° for 40 minutes. Remove from oven and place miniature marshmallows on top of hot cake. Let marshmallows melt then frost with icing (below) while still warm.