In a saucepan, combine the Coca-Cola, oil, 1 stick of butter, and 3 tablespoons of cocoa powder. Bring the mixture to a boil over medium heat, whisking constantly until fully combined.

In a large mixing bowl, mix together the sugar, flour, and salt until fully combined.

Once the Coca-Cola mixture is boiling, pour it over the dry ingredients and beat until smooth.

Add the eggs, buttermilk, baking soda, and vanilla extract, and beat until fully incorporated.

Pour the batter into a greased and floured 13x9 inch baking pan.

Bake in a preheated oven at 350°F (175°C) for 20 to 25 minutes, or until a wooden skewer inserted into the center comes out clean.

Remove from the oven and let the cake cool for about 10 minutes in the pan.

In a saucepan, combine 1 stick of butter, 3 tablespoons of cocoa powder, and 6 tablespoons of cream or milk. Heat while stirring until the butter is melted.

Remove from the heat and whisk in the vanilla extract, then gradually beat in the confectioner’s sugar until the frosting is smooth.

Spread the warm frosting evenly over the slightly cooled cake while it’s still in the pan.