• Preheat oven to 350 degrees F.  Spray a 9x13-inch baking dish with cooking spray or line two 9-inch cake pans with parchment paper circles and spray sides with cooking spray.
• Whisk flour, sugar, cocoa powder, baking soda, baking powder, and salt together in a large bowl.  
• Add the buttermilk, oil, eggs, and vanilla and mix on medium speed using a hand mixer or stand mixer until combined.  
  
• Add the hot water and mix on low speed until combined.  Scrape bottom and sides of the bowl and mix on medium speed for 30 seconds.
• Pour into the baking dish or divide evenly between cake pans.  Bake for 32-37 minutes for a rectangular cake, or 30-35 minutes for 9-inch round cake pans until a toothpick inserted into the center of the cake comes out clean.  
• Cool completely before frosting.

Chocolate Frosting

• In a large bowl, beat the butter, salt and cocoa powder together until creamy and smooth.
• Gradually add the powdered sugar 1 cup at a time, with 1 tablespoon of the cream at a time, mixing between each addition and scraping down the sides of the bowl as needed.  Add additional cream as needed and continue to beat for about 5 minutes until light and fluffy.