Preheat oven to 350 degrees F. Grease and flour 2 9 inch round cake pans.
Mix cocoa and boiling water together in a small bowl or large measuring cup; set aside.
Mix buttermilk, oil, eggs and vanilla together in a large mixing bowl
Mix sugar, flour, salt and baking soda together in a second mixing bowl.
Mix dry ingredients into buttermilk/oil mixture and stir well with a heavy duty rubber scraper. Mix in cocoa mixture – ours was still very warm when we stirred it in. Stir until well mixed (though batter might have a few lumps). Don’t overbeat the flour.
Pour batter into pans and bake at 350 for 30 minutes or until a cake tester comes out clean. Let cool in pans for 10 minutes. Turn from pans and cool.
Frosting: Melt both chocolates in microwave, double boiler or bowl set over barely simmering water; set aside to cool. Beat the butter until creamy. Make sure it’s not too cold or it might cause the chocolate to re-solidify. Stir in the cocoa power, about a cup of the powdered sugar, and the cooled melted chocolate. Scrape sides of bowl and beat well; add remaining powdered sugar, then add room temperature milk, vanilla and salt, beating and scraping until you get the consistency you like.