Preheat the oven to 350 degrees and grease a 9x13 baking pan with nonstick cooking spray.

In a medium saucepan, combine the Coca Cola, butter, and cocoa powder. Cook over medium heat until the butter is melted and the mixture just begins to boil, stirring occasionally. When the mixture begins to boil, remove it from heat and allow it to cool for around 10 minutes.

While the soda is reducing, whisk together the flour, sugar, baking soda, and salt in a large bowl.

Pour the soda mixture over the flour mixture and stir to mix until just moistened.

Add the eggs, buttermilk, and vanilla extract and whisk until well blended.

Pour the batter into the prepared pan and bake for 25 to 30 minutes, until a toothpick inserted in the middle comes out clean.

While the cake is baking, prepare the glaze by warming the Coca Cola in a medium saucepan over medium heat until the mixture begins to come to a boil. Continue cooking until the soda is reduced to around 1/2 cup.

Stir in the butter, vanilla extract, and cocoa powder until the cocoa is dissolved and the butter is melted.

Remove the pan from heat and add the powdered sugar, stirring until the mixture is smooth.

Pour the glaze over the top of the warm cake.

Allow the cake to cool completely before slicing and serving.