FOR THE CAKE: Preheat oven to 350 degrees F. Grease and flour a 9 x 13-inch baking pan.
In an electric mixer, whisk together the flour, sugar, baking soda, salt and cinnamon.
In a small saucepan, stir together the cola, butter and cocoa powder over medium-high heat and bring to a boil, stirring occasionally. Pour into the flour mixture and stir with a rubber spatula until moistened.
In a small bowl, whisk together the eggs, buttermilk and vanilla until blended. Add to the batter, whisking constantly.
Pour the batter into the prepared pan and bake until a toothpick inserted into the center comes out clean, 35 to 40 minutes.
FOR THE FROSTING: About 5 minutes before the cake is finished, begin the frosting. In a small saucepan, bring the cola, cocoa powder and butter to a boil. Remove from heat. Whisk in the powdered sugar and vanilla until smooth. Pour immediately over the hot cake in an even layer. Leftovers should be covered and stored at room temperature or in the refrigerator for up to 4 days.
Recipe by Dallas Duo Bakes at http://dallasduobakes.com/2015/04/coca-cola-cake.html