In a mixing bowl, combine flour, sugar, baking soda and salt.
In a saucepan over medium heat, melt butter. Add cocoa. Stir together.
Add hot water. Bring to a boil for 30 seconds. Turn off heat.Pour into flour mixture and stir.
Add vanilla, buttermilk, and eggs and fold into mixture. Pour into sheet cake pan (approximately 18 x 13). Bake at 375 degrees for 15-18 minutes.
While cake is baking, make the chocolate frosting. Melt butter in a saucepan. Add cocoa. Stir to combine, then turn off heat. Add the powdered sugar, vanilla, and buttermilk or heavy cream. Stir together. Pour over warm cake.