Instructions

1. Take whatever you need out of the fridge so that all ingredients can come to room temperature.
2. Preheat the oven to gas mark 350°F, putting in a baking sheet as you do so, and line a small loaf tin with greased foil – making sure there are no tears – and leave an overhang all round. Or use a silicone tin.
3. Put the flour, baking soda, cocoa, sugar, butter, eggs, vanilla and sour cream into the processor and blitz until it’s 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.
4. Scrape and pour this beautiful batter into the prepared loaf tin and slide into the oven, cooking for about 1 hour. When it’s ready, the loaf will be risen and split down the middle and a cake-tester, or a fine skewer, should come out clean. But this is a moist cake, so don’t be alarmed at a bit of stickiness in evidence; rather, greet it like a friend.
5. Not long before the cake is due out of the oven (when it’s had about 45-50 minutes) put the syrup ingredients of cocoa, water and sugar into a small saucepan and boil for 5 minutes. You may find it needs a little longer – what you want is a reduced liquid, that’s to say a syrup, though I often take it a little further, so that the sugar caramelizes and the syrup has a really dark, smokey chocolate intensity.
6. Take the cake out of the oven and sit it on a cooling rack and, still in its tin, pierce here and there with a cake tester. Then pour the syrup as evenly as possible over the surface of the cake. It will run to the sides of the tin, but some will have been absorbed in the middle.
7. 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. Now take your bar of chocolate and cut with a heavy sharp knife, so that it splinters and flakes and falls in slices of varying thick- and thinness. I’ve specified a weight, but judge it by eye – when you think you’ve got enough to scatter over the top of the loafcake, stop slicing. Sprinkle these chocolate splinters over the top of the sticky surface of the cake.

Recipe adapted from Nigella Lawson