To make cake: Heat oven to 400º. Spray a 13x9x2-inch pan with cooking spray. Whisk the buttermilk and baking soda in a small nonreactive bowl. (The buttermilk will start to swell.)
In a medium saucepan, combine 2 sticks butter, 1 cup Coca Cola, and the cocoa powder and bring to a boil.
Meanwhile, place flour and 2 cups superfine sugar in a large mixing bowl fitted to an electric mixer and use the whisk attachment to sift them together. Pour the hot Coke mixture into the mixing bowl and mix on moderately low-speed until thoroughly combined, about 2-3 minutes.
Mix in the eggs, vanilla extract, and buttermilk mixture. Mix well to combine. Batter will be thin. Pour into prepared pan. Bake for 20 minutes, or until a toothpick inserted in the center comes out clean. Let cake cool for 5 minutes.
To make Coca Cola Frosting: Combine the confectioners’ sugar (3 3/4 cups plus 2 tablespoons) and a large pinch of salt in a large mixing bowl. In a small bowl whisk together butter and melted chocolate until smooth. (I melted them to together in a small saucepan over low heat.) Mix vanilla extract into chocolate mixture.
Pour the chocolate mixture over the confectioners’ sugar and add the 6 tablespoons of Coca Cola. Beat on low-speed for 1 ½ to 2 minutes, just until combined and smooth. Scrape down sides of the bowl to ensure everything is mixed evenly. Pour frosting over cake and use a rubber spatula to smooth it out.
To make Chocolate Cookie Frosting: Place 4 cups confectioners’ sugar and salt into a mixing bowl of an electric mixer fitted with a paddle attachment. Add melted chocolate, vanilla extract, and milk. Beat on medium-low speed for about 2 minutes.
Blend in butter 2 tablespoons at a time, beating just until incorporated. Increase speed to medium and beat until smooth, about 1-2 minutes. Scrape down sides of bowl periodically to keep frosting smooth.
Note: If frosting is too thick, add additional milk 2 teaspoons at a time (no need to warm this milk). If too thin, add additional confectioners’ sugar. Use a pastry bag to pipe frosting onto cake. Garnish with walnuts if desired.