Preheat your oven to 170°C/gas mark 3/325ºF. Grease a 22 or 23 cm/ 9inch springform tin with a little oil and line the base with baking parchment.
Measure and sift the cocoa powder into a bowl or jug 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 ground almonds (or flour) with the bicarbonate of 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 ground almond (or flour) mixture.
Scrape down, and stir a little with a spatula, then pour this dark, liquid batter into the prepared tin. Bake for 40-45 minutes or until the sides are set and the very centre, 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 tin, and then ease the sides of the cake with a small metal spatula and spring it out of the tin. Leave to cool completely or eat while still warm with some ice cream, as a pudding.