Cake: In a saucepan melt butter with water and cocoa powder, bring to a boil and remove from heat. In mixing bowl combine flour, sugar, and salt. Add warm chocolate mixture to flour combine. In a separate bowl whisk together buttermilk, egg, and baking soda. Add this to cake batter, mix well. Pour into a 15x12 sheet pan. Bake at 350 degrees for 25 minutes or until toothpick comes out clean.
Dollup marshmallow fluff on top of cake as soon as it's out of the oven. Let the marshmallow sit a minute and start to melt, then it will be easier to spread.
Icing
While cake is baking: Add butter and cocoa to saucepan over medium heat melt butter and whisk the two ingredients together, when butter is melted and mixture starts to bubble remove pan from stove top add milk vanilla, and powdered sugar whisk until smooth. Slowly, pour over marshmallow covered cake making swirl patterns in the icing with spatula so that the marshmallow shows through.