Hostess with the Mostess Chocolate Snack Cake

Name: Hostess with the Mostess Chocolate Snack Cake
Prep time: 45 min
Cook time: 15 min
Total time: 1 h
Cuisine: European
Ingredients: 9 X 13 baking dish, covered preferred; 12 X 18 rimmed baking sheet; Unrimmed cookie sheet larger than 9 X 13; Offset spatula; Piping bag and Wilton #3 tip; metal ruler (helpful when cutting); mini offset spatula (very helpful when lifting cake squares); 1 cup confectioner’s sugar; 1/2 tsp vanilla; 1 Tbsp milk; 3 tbsp unsalted butter, softened; 10 oz bittersweet chocolate, finely chopped; 1 Tbsp light corn syrup; 1/3 cup heavy cream; 2 cups confectioners’ sugar; 6 Tbsp unsalted butter, room temperature; 7 oz jar marshmallow crème; 1 tsp vanilla; 4 Tbsp heavy cream; ¾ cup natural unsweetened cocoa; 2/3 cup hot water; 1 cup buttermilk; 1 ¾ cups all-purpose flour; 1 ½ teaspoon baking soda; ½ teaspoon kosher salt; ¾ cup unsalted butter, room temperature; 1 cup granulated sugar; ½ cup light brown sugar, firmly packed; 3 large eggs; 2 teaspoons vanilla