Preheat oven to 350 degrees Fahrenheit.
Line one 18” x 13” sheet cake pan with parchment paper (leave parchment paper slighter higher than the sides of the pan so it’s easier to lift the cake out after). Spray lightly with non-stick cooking spray then set aside.
In a large bowl, sift together flour, sugar, unsweetened cocoa powder, baking soda, baking powder and salt. Sift together to remove any lumps (you may need to sift this twice). Set the dry ingredients aside for now.
In a separate large bowl, whisk together buttermilk, eggs, vegetable oil and vanilla until well combined.
Stir dry ingredients into wet ingredients just until mixed. Do not overmix. Be sure to stir these in by hand – do not use electric beaters for this.
Pour the boiling water into the cake batter and stir until well mixed. Use a spatula to scrape down sides and bottom of bowl.
Pour cake mix into the prepared sheet cake pan. This will be very full so be careful when transferring it to the oven.
Bake in oven for 26 to 27 minutes or until toothpick inserted in middle comes out clean.
Remove cake from oven and let cool completely in pan before frosting.