Spray a 13" x 9" baking pan with baking spray and dust with flour.
In a bowl mixed together flour, white sugar, baking soda, cinnamon, and salt. Set aside.
In a medium saucepan bring butter, Coke, buttermilk, brown sugar and cocoa powder to a boil over medium-high heat whisking constantly.
Remove chocolate from heat and whisk butter mixture into flour mixture until smooth.
Add eggs and vanilla and mix until smooth.
Spread batter into a pan and bake for 35 to 45 minutes until a toothpick comes out clean.
FROSTING
In a medium saucepan, bring butter Coke, and cocoa powder to a boil over medium heat whisking constantly. Remove and gradually whisk in powdered sugar until smooth and pour over cake.