Preheat oven to 350 degrees F (175 degrees C). If using layers, grease and line the pans with parchment paper. If using a 9 X 13 pan, grease and dust with cocoa powder. Sift together the cocoa, flour, baking soda and salt. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time. Alternately beat in the flour mixture and the buttermilk, mixing just until incorporated. Add the boiling water and beat for one full minute.
Bake in the preheated oven for 30 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.