Preheat oven to 350°. Spray an 18×13-inch baking sheet with baking spray with flour.
In a large bowl, combine flour and granulated sugar.
In a small saucepan, melt 2 cups butter over medium heat. Whisk in ½ teaspoon salt and ¼ cup cocoa until well combined. Stir in 1 cup boiling water.
In a large bowl, whisk together buttermilk, beaten eggs, baking soda, flour mixture, and 1 teaspoon vanilla. Pour chocolate mixture into buttermilk mixture, stirring until combined. Pour into prepared pan.
Bake for 20 to 25 minutes. Let cool for 10 minutes.
Meanwhile, in a large saucepan, melt remaining 1¾ cups butter over medium heat. Add remaining ¼ cup cocoa, stirring to combine. Remove from heat. Stir in milk, remaining 1 teaspoon vanilla, confectioners’ sugar, and remaining ¼ teaspoon salt, working quickly so icing doesn’t harden.
Reserve ½ cup frosting. Pour remaining frosting over warm cake. While icing is warm, sprinkle with marshmallows and pecans. Drizzle with reserved ½ cup frosting. Serve warm or let cool completely.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/mississippi-mud-cake-2/