• Preheat the oven to 325°F/160°c.  Line an 9 inch x 13 inch sheet pan with parchment paper and set aside.
• Add the all purpose flour, granulated sugar, cocoa powder, bakingpowder, baking soda and salt to a large mixing bowl.  Whisk until well combined and set aside.
• Add the buttermilk, oil, egg and vanilla extract to another largemixing bowl.  Whisk until wellcombined. 
• Pour the wet ingredients into the bowl with the dry ingredients.  Whisk until just combined.  Add the hot water, and whisk until justcombined, starting the whisking gently to ensure the hot water doesn’t splashout of the bowl.
• Pour the cake batter into the prepared sheet pan.  Bake for 35 minutes, or until cookedthrough.  Set aside to cool completely inthe sheet pan.

Buttermilk Chocolate Frosting

• To make the buttermilk frosting, add the softened butter to the bowl ofa stand mixer.  Beat using a wire whiskattachment on a high speed for 2 minutes or until smooth.
• Add the powdered sugar to the butter, 1 cup at a time, beating on a lowincreasing to high speed until well incorporated.  Repeat with the cocoa powder.
• Add the buttermilk, vanilla extract and salt, and beat on a mediumspeed until well combined, scraping the sides of the bowl as necessary.  Increase the speed to medium-high and beatfor 1-2 minutes or until smooth and fluffy.
• Spread the frosting over the cooled cake before slicing and serving.