Preheat the oven to 350 degrees F.
Grease a 13 by 9 by 2-inch pan with butter.
In a large mixing bowl, stir together the cocoa and the melted butter. Add the eggs, flour, granulated sugar, salt, and vanilla. Beat well with a hand-held electric mixer. Add nuts, stirring with a rubber spatula. Pour batter into the prepared pan. Bake for 35 minutes.
While cake is baking, prepare the frosting. Beat the brown sugar and butter. Add the cocoa and beat. Stir in the scalded milk and vanilla together in a mixing bowl and beat with the hand-held mixer until smooth. Set aside.
Remove the cake from the oven, and pour the marshmallows over the cake while it is still hot. Pour the frosting over the marshmallows. Let cool slightly before cutting into pieces.