The Altitude Adjustment Section
This is wet and gooey, almost like a lava cake. It's super-rich and delicious. Not really very attractive so if you are going to plate it, prepare some garnish like whipped cream, or home made ice cream, or caramel sauce, or ... well, you get the idea.
1 room temperature unsalted butter
1 2/3 cup dark brown sugar
2 large eggs, beaten
1 teaspoon vanilla extract
4 ounces semi sweet chocolate, melted
1 1/3 cup all-purpose flour
1 teaspoon baking soda
1 cup plus 2 tablespoons boiling water
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 (Nigella's term) cake: use parchment or one of those loaf-pan-shaped paper liners. Cream the butter and sugar with an electric 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, (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 smaller cake or muffin pan) and bake for 30 minutes.
Turn the oven down to 325 degrees and continue to cook for another 15 minutes. The cake will still be a bit squidgy (also Nigella's term) 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. Nigella says she often leaves 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." Damp? Nigella, really how unappealing!