How to Make It
Place the 4 cups of Coca Cola for the Frosting in a medium sized pot and boil over medium-high heat until the liquid is reduced to 1/3 of a cup, about 1 hour and 5 minutes. The liquid will be thick and syrupy. Set aside to cool to room temperature.
While the Coca Cola for the Frosting is boiling, make the Cake: Preheat oven to 350°F. Grease and line two 9-inch round cake pans with parchment paper. In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix briefly to combine. Scatter butter across the surface of the dry ingredients and paddle on low until completely incorporated, about 2 minutes. In a separate bowl, whisk together 1 cup Coca Cola, vegetable oil, buttermilk, eggs, and vanilla. Add the liquid ingredients in 3 additions to the dry ingredients, beating the mixture on medium-low for 2 minutes between each addition. Divide batter evenly among cake pans and bake until a toothpick inserted comes out clean, 30 to 35 minutes. Allow to cool completely before removing from pans, about 1 hour.
In a clean bowl, beat together butter and cream cheese until well combined. Add powdered sugar and salt and beat on low until incorporated. Increase the speed to medium-high and beat for 4 minutes until shiny and satiny. Use a rubber spatula to scrape the room temperature Coca Cola syrup into the frosting. Beat for another 2 minutes until well incorporated. Scrape down the sides of the bowl to incorporate any syrup that sticks to the bowl, and beat again for 1 minute to combine.
Place one cake layer on a cake plate. Spread 1/2 cup frosting evenly across the surface. Add the second cake layer. Take 1 cup of frosting and spread it evenly over the entire cake. Refrigerate for 30 minutes before taking the rest of the frosting and spreading it over the cake, using a spoon to create decorative swooshes in the frosting.