Preheat oven to 350°F. Spray a 9x13 baking pan with nonstick spray.
Whisk together flour, granulated sugar and baking soda in mixing bowl. Set aside.
In a small saucepan over medium heat, melt 1 cup butter with 5 tablespoons cocoa and 1 cup Coca Cola. Bring to a boil, whisking together as it melts.
Pour the warm chocolate mixture over the flour mixture and whisk until thoroughly combined. Add buttermilk, eggs and vanilla and continue whisking until combined.
Pour batter into prepared 9x13 pan and bake in preheated oven for 40 minutes.
During last 10 minutes of baking prepare frosting.
Set the marshmallow cream aside. Sift 4 cups of confectioners' sugar into a large mixing bowl. In a small saucepan over medium heat melt 1/2 cup butter, 3 tablespoons cocoa powder and 9 tablespoons Coca Cola. Whisk together and bring to a boil. Remove from heat and whisk in 1 teaspoon vanilla.
Pour the warm chocolate mixture over the sifted confectioners' sugar. Whisk together until completely combined.
Remove cake from oven and immediately drop dollops of marshmallow cream onto the top of the hot cake. Allow it to sit for a minute or so. Carefully spread the marshmallow cream across the top of the cake. Slowly drizzle the warm chocolate frosting over marshmallow layer.
Allow cake to cool completely before serving. Tastes even better the next day!
If you don't like marshmallow cream, you can try using mini marshmallows. However, mini marshmallows will leave the top looking lumpy. If you'd like, you can omit the marshmallow altogether and stick to just the chocolate frosting.