Make sure all the ingredients are at room temperature.
Preheat oven 350 F, Line 9"x13" pan with parchment paper and grease the sides.
In a large bowl, sift the flour, sugar, salt and baking soda and stir to combine. Set aside.
In a medium saucepan, while constantly stirring, bring to a boil the cocoa powder, water and butter. Remove from heat and pour into a bowl of a standing mixer.
Add the chocolate mix into the flour mixture.Using a hand mixer (or a standing mixer) mix in the eggs and buttermilk just until combined. (batter will be thin)
Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick is clean when inserted into the center of the cake.
Immediately when removing cake from the oven sprinkle the marshmallows over the cake and allow the cake to cool, the Marshmallows will melt as cake is releasing heat during the cooling process.
Once the cake has cooled for a while, drizzle the chocolate topper using a spoon or a piping bag.
Place all the ingredients into a saucepan over a low-medium heat and stir until smooth.