In a medium saucepan, combine 250g butter or margarine, 1/4 cup cocoa and water. Cook over medium heat until butter is melted.
In a large mixing bowl stir together sugar, flour, bicarb soda and cinnamon. Add cocoa mixture to flour mixture and blend. Beat together eggs and buttermilk and blend into batter thoroughly. Stir in 1 teaspoon vanilla. Pour batter into ungreased 20x30cm pan.
Bake for 20 minutes at 180 degrees C.
To make icing: Five minutes before cake is done, combine 125g butter, 1/4 cup cocoa and milk in a saucepan. Bring to a boil and simmer about 3 minutes; remove from heat. Stir in icing sugar and 1 teaspoon vanilla, blending until smooth. Spread immediately over hot cake in pan. Sprinkle with nuts.