Preheat the oven to 350 degrees and prepare a 9x13 baking dish with non-stick cooking spray.
In a large bowl, whisk together the all purpose flour, white granulated sugar, baking soda and salt until fully incorporated. Set aside.
In a small saucepan, simmer the entire can of coca cola over medium heat until it reduces to about ½ cup. Use a measuring cup to accurately measure and discard the extra.
Add the unsalted butter and baking cocoa and whisk until the butter is melted and the cocoa is completely dissolved. Remove from heat and cool for 5 minutes.
Once cooled, add the Coca Cola mixture to the flour mixture. Add the eggs, buttermilk and vanilla and use an electric mixer on medium speed to blend until well combined.
Pour the cake batter into the prepared cake pan. Bake for 30 minutes or until a toothpick or knife inserted in the center comes out clean.
While the cake is baking, prepare the frosting.
In a small saucepan over medium heat, reduce the second can of Coca Cola by simmering over medium heat until reduced to a ½ cup. Use a measuring cup to accurately measure and discard any extra.
Add the butter, vanilla and cocoa to the mixture and whisk until butter is fully melted and mixture is smooth. Remove from heat.
Add the powdered sugar ½ cup at a time, whisking together until dissolved. Repeat until all four cups are incorporated.
Pour the frosting over the baked cake. Allow the cake to cool before serving.
Optional: Serve with ice cream, whipped topping, chocolate syrup or cherries for an extra special presentation.