In a mixing bowl, mix together the dry ingredients: sugar, cocoa powder, baking soda, baking powder and salt.
In a microwave safe bowl add the buttermilk and water. Place in the microwave for 2 minutes until hot.
Pour the mixture into the dry ingredients and stir until combined. Add the eggs and oil.
Spray a 9x13 pan with cooking spray. Pour the batter into the pan evenly. Place in the oven for 25-30 minutes, or until a toothpick comes out clean. While the cake is baking, make the frosting.
For the frosting, place the butter in a mixing bowl. Beat with an electric hand mixer until smooth. Add the cocoa powder, buttermilk, vanilla and salt and beat until smooth. Add the powdered sugar one cup at a time until the frosting is the right thickness.
Once the cake is done baking, remove from the oven and let cool for 5-10 minutes. Frost with the frosting and serve slightly warm with vanilla ice cream.