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.