Preheat your oven to 325 degrees F. Grease your springform pan with a little oil and line the base with parchment paper.
Measure and sift the cocoa powder into a bowl or pitcher and whisk in the boiling water until you have a smooth, chocolatey, still runny (but only just) paste. Whisk in the vanilla extract, then set aside to cool a little.
In another smallish bowl, combine the almond meal (or all-purpose flour) with the baking soda and pinch of salt.
Put the sugar, olive oil, and eggs into the bowl of a freestanding mixer with the paddle attachment (or other bowl and whisk arrangement of your choice) and beat together vigorously for about 3 minutes, until you have a pale-primrose, aerated, and thickened cream.
Turn the speed down a little and pour in the cocoa mixture, beating as you go, and when all is scraped in, you can slowly tip in the almond meal (or all-purpose flour) mixture.
Scrape down, and stir a little with a spatula, then pour this dark, liquid batter into the prepared pan. Bake for 40-45 minutes or until the sides are set and the very center, on top, still looks slightly damp. A cake tester should come up mainly clean but with a few sticky chocolate crumbs clinging to it.
Let it cool for 10 minutes on a wire rack, still in its pan, and then ease the sides of the cake with a small metal spatula and spring it out of the pan. Leave to cool completely or eat while still warm with some ice cream, as a dessert.
Reprinted from the book Nigellissima by Nigella Lawson. Copyright (c) 2013 by Nigella Lawson. Photographs copyright (c) 2013 by Petrina Tinslay. Published by Clarkson Potter, a division of Random House, Inc.