Prepare either a 13 x 9" pan or 2 8" round pans with cooking spray.
In a small saucepan bring butter, water and cocoa to a boil.
In a separate bowl, combine the flour, sugar, baking powder, baking soda and salt.
Add cocoa mixture.
Combine the buttermilk and the vanilla, then add to the batter and mix well.
Beat in the eggs.
Pour into prepared pans.
Bake for 23-27 minutes or until a toothpick inserted into the center comes out clean.
For the frosting:
In a bowl whisk together the cocoa and the sugar and set aside.
Then in the bowl of a stand mixer fitted with a paddle attachment, cream the butter until smooth.
Add the cocoa mixture, alternating with the whipping cream until mixture is creamy and blended well.
Add the vanilla.
Gradually add cream until you have the desired spreading consistency. Do not add a lot at once or your frosting will get soupy.
Notes
Note: For this cake I garnished with berries and I used about 1/4 cup of frosting which I heated in the microwave and drizzled over the top of the berries.