Preheat the oven to 350 degrees.

In a medium saucepan, melt the butter, Coca-Cola and cocoa. Bring the mixture to a boil, then remove the saucepan from the heat and allow it to cool slightly.

In a large mixing bowl, combine the flour and sugar.

Pour the cooled cocoa mixture over the top.

Add the buttermilk, eggs, baking soda and vanilla extract. Whisk or beat until the ingredients are well combined.

Pour the cake batter into a 9 x 13 x 2-inch cake pan or Pyrex dish. Bake for 45-50 minutes until a toothpick inserted near the center comes out clean. Allow the cake to cool completely before icing.

In a large mixing bowl, cream the butter until it is light a fluffy. Add Coca-Cola, powdered sugar, vanilla and baking cocoa. Mix on low until everything is wet, then increase the speed to medium and mix until everything is evenly incorporated.

Pour the icing over the cooled cake. Using a spatula, evenly spread the icing over the top and to the edge of the cake pan. Cover the cake with a lid of plastic wrap, then refrigerate for two hours to allow the icing to set.