Preheat the oven to 325 degrees. Grease a pan very well with butter and set aside. (A bundt, 9x13, or (2) 8-9" round pans will work well.) Combine cocoa and hot water in a large glass measuring cup or small bowl and whisk to combine. Place the butter and the sugar in a mixing bowl and beat until smooth and creamy. Add the cocoa mixture and stir to combine. Add the eggs and vanilla and stir again. Add the sour cream and stir once more.

Add the dry ingredients to the wet mixture and mix well. Slowly add the boiling water while mixing. The batter will be very thin and watery. Pour into the prepared pan and bake for 25-42 minutes depending on pan. Test for doneness with a toothpick, the bundt cake took the full 42 minutes to bake. The cake should no longer look wet and should leave moist crumbs on the toothpick. Let the cake cool completely before icing.

To make the icing, combine all ingredients in a heavy bottomed sauce pan over medium heat. Whisk while the mixture thickens and bring it almost to the boiling point. This should take about ten minutes. While the icing is hot, pour over the cake and spread it lightly across the surface. Let it cool enough to stop steaming before placing it in the refrigerator.

Cover the cake and chill overnight before serving. It is delicious served warm (because some of us were impatient), however it is best served cold. Enjoy!