Preheat oven to 350 degrees F. Liberally grease a 9x13" rectangular baking pan with cooking spray; set aside.

In a medium saucepan, add in the butter, cocoa powder, and can of Coke and bring to a boil over medium heat, whisking occasionally until smooth. While Coke mixture melts, whisk together the granulated sugar, flour, baking soda and salt in a large bowl until combined. Pour the melted Coke mixture into the flour mixture and combine well.

Add in the buttermilk and gently whisk in to combine. Crack in the eggs and stir in the vanilla, whisking until cohesive and smooth. Pour into the prepared pan and bake for approx. 25-30 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs.

During the last 15 minutes of the cake baking, make your frosting. In a small saucepan, boil the second can of Coke for 12-15 minutes or until reduced by half and thickened slightly. Whisk in the butter to melt.

In a medium bowl, whisk together the cocoa powder and powdered sugar. Pour in the Coke/butter mixture and whisk until smooth. If adding chopped pecans, do so now. Pour the frosting over the still-hot cake and let set at room temperature for about an hour before cutting into squares.