• Preheat the oven to 350°. Grease a 13x9-in. baking pan and set aside.
• In a large bowl, whisk flour, sugar, baking soda, salt and cinnamon, set aside.
• In a small saucepan, combine Coca-Cola, butter and unsweetened cocoa.
• Bring just to a boil over low heat, stirring occasionally.
• Add to dry ingredients, stirring until moistened.
• In a small bowl, whisk eggs, buttermilk and vanilla until blended then add to flour mixture, whisking constantly.
• Transfer to prepared pan.
• Bake 25-30 minutes or until a toothpick inserted in center comes out clean.

For the glaze

• Approximately 15 minutes before the cake is done, make the glaze.
• In a small saucepan, bring Coca-Cola to a boil.
• Cook 12-15 minutes or until liquid is reduced to ½ cup.
• Stir in butter and unsweetened cocoa until butter is melted.
• Remove from heat. Add confectioners (powdered) sugar and stir until smooth.
• Pour immediately over hot cake.