To begin, preheat your oven to 350 degrees. Lightly grease a 9 x13 baking pan. Set it aside for now.
In a measuring cup, combine the buttermilk, eggs, and vanilla. Whisk together and set aside.
In your mixing bowl, combine the flour, baking soda, sugar, salt, and cinnamon. Stir together and set aside for now.
In a saucepan, combine the cola, butter, and baking cocoa. Heat these over medium low heat, stirring occasionally, until the butter is all melted. Remove it from the heat.
Pour the mixture into your mixing bowl and mix until smooth. Add in the buttermilk mixture, and again, beat until it is no longer lumpy. Pour the batter into your prepared cake pan.
Bake this for 35-40 minutes or until the cake tests done in the center with a wooden tooth pick.
For the Icing:
As soon as the cake is done baking, prepare the icing. In a saucepan, combine the butter, baking cocoa, half & half, and vanilla. Stir these over medium heat until the butter is melted.
Add in the powdered sugar and whisk until smooth.
Pour it over the top of the hot cake.
Serve the cake warm or room temperature with a scoop of vanilla ice cream.