Preheat oven to 190 degrees C. Grease a Swiss roll tin.
Bring 250g butter, water and 4 tablespoons cocoa to a boil in a large saucepan. While still hot, remove from heat and add 2 cups flour, 2 cups white sugar and salt. Mix well. Beat in eggs, sour cream and bicarb soda. Do not beat too long.
Pour batter into greased tin. Bake for 20 to 25 minutes or until a toothpick inserted into centre of cake comes out clean.
To Make Icing: Bring 125g butter or margarine, 4 tablespoons cocoa and 5 tablespoons milk to a boil. Remove from heat. While still hot add icing sugar, vanilla and nuts. Beat well. Ice cake while icing is still hot.