Preheat your oven to 350°F (175°C). Grease and flour a large baking sheet measuring 18 by 13 inches.

In a large bowl, combine the flour, sugar, baking soda, and salt, stirring thoroughly to mix.

In a saucepan, melt the butter over medium heat. Once melted, stir in the buttermilk and cocoa powder, bringing the mixture to a simmer.

Pour the hot mixture into the flour mixture and mix until just combined, ensuring not to overmix.

Add the eggs and vanilla extract, mixing until smooth and well-blended.

Carefully stir in the boiling water until you have a smooth batter; it will be quite thin.

Pour the batter into the prepared baking sheet and bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.

For the frosting, melt the butter in a saucepan over medium heat. Add the cocoa and milk, bringing it to a boil.

Remove from heat and whisk in the powdered sugar and remaining vanilla until smooth and glossy.

Immediately pour the frosting over the warm cake, spreading it evenly. Let cool completely before serving.