Preheat the oven to 350°F. Coat a 13- x 9-inch baking dish or baking pan with nonstick cooking spray. Combine the flour, granulated sugar, and baking soda in a large bowl.
Combine 1 cup of butter, 1 cup Coca Cola, the buttermilk, and ¼ cup cocoa in a medium saucepan. Cook over medium heat, whisking occasionally, until the mixture comes to a boil. Add to the flour mixture and stir until smooth.
Whisk in the eggs, one at a time, whisking until the yellow disappears after each addition. Whisk in the vanilla.
Pour the batter into the prepared pan. Bake for 32 to 35 minutes or until a toothpick inserted in the center of the cake comes out clean.
Meanwhile, make the frosting: Bring the butter, cola, and cocoa to a boil in a medium saucepan over medium-high heat. Remove from the heat and gradually whisk in the powdered sugar until smooth.
Pour the warm frosting over the hot cake. Let stand at room temperature until cooled. Cut into squares to serve.