Lightly spray an 18×13 baking sheet with cooking spray and set aside.Preheat your oven to 350°.
In a large mixing bowl combine your flour, sugar, and salt.
In a large sauce pan over medium-low heat melt your butter. Add your cocoa powder and whisk together.
Add boiling water and whisk it into the mixture.
In a separate small bowl combine your buttermilk, eggs, baking soda and vanilla and whisk together. Stir buttermilk mixture into chocolate mixture and whisk together. Mix with dry ingredients (flour, sugar, and salt).
Pour into sheet cake pan and bake at 350° degrees for 20 minutes.Remove from the oven and poke with forks (so that the yummy frosting goes down into the cake just a little bit and makes it even that much better).
While your cake is baking start your frosting by melting your butter in a large saucepan over med-low heat. Add your cocoa and whisk together. Turn off heat. Add your milk, vanilla and powdered sugar (sift it in while whisking instead of just dumping all of it in at once). Stir until no lumps remain.
Pour your frosting over your cake and let cool for about 10-15 minutes before serving.