Take whatever you need out of the refrigerator so that all ingredients can come room temperature.
Preheat the oven to 325 degrees F, putting in a baking sheet as you do so.
Put the flour, baking soda, cocoa, sugar, butter, eggs, vanilla, and sour cream into the processor and blitz until a smooth, satiny brown batter. Scrape down with a rubber spatula and process again while pouring the boiling water down the funnel. Switch it off, then remove the lid and the well-scraped double-bladed knife and, still using your rubber spatula, stir in the chocolate chips or morsels.
Scrape and pour the batter into the prepared loaf tin and put into the oven, cooking for about 1 hour. When ready, the loaf will be risen and split down the middle and a cake-tester will pretty well come out clean.
Not long before the cake is due out of the oven (when it has had about 45 to 50 minutes), put the syrup ingredients of cocoa, water and sugar into a small saucepan and boil for about 5 minutes, to give a thick syrup.
Take the cake out of the oven and sit it on a cooling rack, still in the tin, and pierce here and there with a cake tester. Pour the syrup over the cake.
Let the cake become completely cold and then slip out of its tin, removing the foil as you do so. Sit on an oblong or other plate. Sprinkle the chocolate splinters over the top of the sticky surface of the cake
Recipe courtesy of Nigella Lawson(Copyright 2004, Feast, Hyperion, All Rights Reserved)