Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large saucepan, combine butter and Coca-Cola. Bring to a boil over medium heat, then remove from heat and stir in cocoa powder until smooth.
In a large bowl, combine flour, sugar, and baking soda. Slowly pour in the chocolate mixture and stir until combined.
Add buttermilk, eggs, and vanilla extract to the batter and mix until smooth.
Pour batter into prepared pan and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
While cake bakes, prepare glaze by melting butter in a saucepan. Remove from heat and stir in cocoa powder, powdered sugar, Coca-Cola, and vanilla extract until smooth.
Pour the glaze over the hot cake immediately after removing it from the oven. Allow to cool before serving.
Notes
This classic Southern cake is perfect for any celebration and pairs wonderfully with a cold glass of milk.