1. Preheat oven to 350 degrees
2. To prepare the cake batter, combine the butter and sugar in a mixing bowl. Beat well until creamy. Add the eggs, 1 at a time, beating thoroughly after each addition.
3. Sift together the flour and cocoa. Fold this into the creamed mixture. Add the chopped nuts and the vanilla. Beat well.
4. Butter the bottom and sides of a 9 X 33 cm baking pan. Add a little flour and shake it around to coat the bottom and sides of the pan. Shake out the excess.
5. Spoon the cake batter into the pan and bake 25 to 30 minutes. Remove from the oven and sprinkle the top with the marshmallows. Return to the oven and bake about 10 minutes, until the marshmallows are melted and starting to brown. Remove from the oven and let cool in the pan about 30 minutes.
6. Meanwhile, prepare the icing. Melt the butter in a saucepan.
7. Sift together the confectioner's sugar and cocoa. Stir this into the butter along with the nuts and milk. Spread this over the cake and let stand until thoroughly cooled.
8. Cut into slices and serve.