1. Preheat oven to 325°F. Grease a 9-inch springform cake pan with a little oil and line base of pan with parchment paper.
2. Sift cocoa powder into a bowl and whisk in boiling water until you have a smooth, chocolaty, still runny (but only just) paste. Whisk in vanilla extract and set aside to let cool.
3. In a small bowl, combine almond meal (or flour) with baking soda and a pinch of salt.
4. Put sugar, olive oil and eggs into bowl of a freestanding mixer with a paddle attachment (or use other bowl and whisk). Beat vigorously until mixture is pale-primrose, aerated, thickened and creamy, about 3 minutes.
5. Turn down mixer speed a little and beat in cocoa mixture. Slowly add almond-meal mixture.
6. Turn off mixer. Scrape down sides of bowl and stir a little with a spatula. Pour batter into prepared cake pan. Bake until sides of cake are set and very center, on top, still looks slightly damp, 40 to 45 minutes. A cake tester should come up mainly clean but with a few sticky chocolate crumbs clinging to it.
7. Let cake cool in its pan on a wire rack for 10 minutes. Ease sides of cake with a small metal spatula and spring cake out of pan. Leave to cool completely or serve warm with ice cream.