Make the cake: Preheat oven to 350°F. Butter and flour a half sheet rimmed baking pan. In a large bowl, whisk together flour, sugar, salt and baking soda. Set aside.
Melt butter in a medium saucepan over medium high heat. Whisk in cocoa blend until smooth, and add water. Bring to a boil. Remove from the heat.
In a medium sized bowl, combine eggs, buttermilk, and vanilla and whisk until combined. Pour in flour mixture and then cocoa mixture and mix until well incorporated. Pour batter into prepared baking sheet and bake until a cake tester inserted into the center comes out clean, about 25 to 30 minutes.
While the cake is baking, prepare the icing: Melt butter in a medium sauce pan over medium high heat. Whisk in cocoa powder until well combined. Remove from the heat then add milk and vanilla. Slowly add confectioners sugar, whisking until smooth. Stir in marshmallows. Pour over warm cake and spread with a spatula.
Recipe by Flavor and Friends at http://flavorandfriends.com/cindys-mom-texas-sheet-cake/