Separate the eggs and beat the egg whites until stiff then set aside. In a large bowl, cream the butter with the sugar. Melt the chocolate in the hot water. Beat in the egg yolks, then the melted chocolate, buttermilk and vanilla.
In a bowl, stir together the flour, bicarbo soda, and salt. Beat into the chocolate mixture. Stir in about 1/3 of the egg whites to lighten the batter and then fold in the rest gently but thoroughly.
Turn the batter into a greased and floured ring tin. Bake in a preheated 180 degrees C oven for 1 hour, or until the cake tests done with a toothpick. Let cool on a rack. Makes 16 servings.