Preheat the oven to 350 degrees.
Lightly grease a 9×13 pan.
Mix the cake mix, cola, butter, milk and eggs until thoroughly mixed.
Once completely mixed, Pour into the pan.
Bake at 350 degrees for 40 to 45 minutes.
Meanwhile make the icing:
In a medium saucepan, melt the butter.
Mix in the cocoa power and cola. Bring just to a boil.
Remove from the heat and stir constantly.
Mix in the powdered sugar until a the icing thickens. Allow it to cool slightly.
Pour on top of the cake.
Let the cake cool completely before serving.