On creating this cake, Nigella Lawson explains, "I remember very strongly wanting to create what we call a loaf cake—and what is generally called a pound cake Stateside—that had a richness and squidginess of texture that this form of cake normally doesn't major in." And she did. Recipe adapted very slightly from How to Be A Domestic Goddess (Hyperion, 2005) —Genius Recipes
Heat the oven to 375°F, put in a baking sheet in case of sticky drips later, and grease and line a 9x5-inch loaf pan. The lining is important as this is a very damp cake: use parchment or one of those loaf-pan-shaped paper liners. This recipe is likely to leave you with a bit of excess batter, so if you'd like to bake it off, have an additional mini loaf pan or muffin tin ready as well.
Cream the butter and sugar, either with a wooden spoon or with an electric hand-held mixer, then add the eggs and vanilla, beating in well. Next, fold in the melted and now slightly cooled chocolate, taking care to blend well but being careful not to overbeat. You want the ingredients combined: You don't want a light, airy mass.
Then gently add the flour, to which you've added the baking soda, alternately spoon by spoon, with the boiling water until you have a smooth and fairly liquid batter. Pour into the lined loaf pan. (Note: Don't let this batter come closer than 1 inch from the rim of the cake pan or it risks overflowing. Pour any excess into a greased mini loaf pan or muffin pan and start testing for doneness at 20 minutes.)
Bake the cake for 30 minutes. Turn the oven down to 325° F and continue to cook for another 15 minutes. The cake will still be a bit squidgy inside, so an inserted cake tester or skewer won't come out completely clean.
Place the loaf pan on a rack, and leave to get completely cold before turning it out. (I often leave it for a day or so: like gingerbread, it improves.) Don't worry if it sinks in the middle: indeed, it will do so because it's such a dense and damp cake.