1. Preheat the oven to 350?F. Butter an 11-by-15-inch pan. (I used a 9 x 13.)
2. Put the cocoa in a medium saucepan and gradually stir in the water. Bring to a boil, add the butter and let melt. Remove from the heat and set aside.
3. Sift the flour, sugar, baking soda and salt together onto a piece of waxed paper. Gradually stir the dry ingredients into the cocoa mixture. Add the eggs, sour cream or buttermilk, and vanilla to the cocoa mixture and stir until smooth.
4. Pour the batter into the prepared pan. Bake for 30 minutes, or until the cake is firm to the touch in the center.
5. While the cake is baking, make the icing: In a medium saucepan, combine the butter, milk and cocoa. Bring to a boil and stir to melt the butter. Add the confectioners’ sugar and beat until smooth. Stir in the nuts and vanilla.
6. Spread the icing over the hot cake (still in the pan). Let cool before cutting.