Preheat oven to 350°. Grease a 13x9-in. baking pan.
In a large bowl, whisk the first five ingredients. In a small saucepan, combine cola, butter and cocoa; bring just to a boil, stirring occasionally.
Add to flour mixture, stirring just until moistened.
In a small bowl, whisk eggs, buttermilk and vanilla until blended; add to flour mixture, whisking constantly.
Transfer to prepared pan. Bake 25-30 minutes or until a toothpick inserted in center comes out clean.
About 15 minutes before cake is done, prepare glaze. In a small saucepan, bring cola to a boil; cook 12-15 minutes or until liquid is reduced to 1/2 cup. Stir in butter and cocoa until butter is melted; remove from heat. Add confectioner's sugar; stir until smooth. Pour immediately over hot cake.
Try using root beer, Dr Pepper or cherry cola.
Sifting the confectioners' sugar helps keep the frosting smooth and lump-free.