Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a large mixing bowl, whisk together flour, sugar, baking soda, and salt.

In a saucepan over medium heat, combine 1 cup cola, 1/2 cup butter, and 1/4 cup cocoa powder. Bring to a boil while stirring constantly.

Pour the hot cola mixture over the dry ingredients and stir until well combined.

In a separate bowl, mix together buttermilk, eggs, and vanilla extract. Add this to the batter and stir until smooth.

Pour the batter into the prepared baking pan and bake for 30 minutes, or until a toothpick inserted into the center comes out clean.

For the frosting, in a saucepan, combine 1/2 cup butter, 1/4 cup cocoa powder, and 6 tablespoons cola. Heat until the butter melts and the mixture is smooth.

Remove from heat and gradually mix in confectioners' sugar until the frosting is smooth and spreadable.

Pour the frosting over the warm cake, spreading it out to cover the entire surface.

Allow the cake to cool before slicing and serving.