Preheat the oven to 160°C / 325°F / Gas mark 3.
Grease a 22 or 23 cm spring form tin with a little oil and line the base with baking parchment.
Measure and sift the cocoa powder into a bowl whisk in the boiling water until you have a smooth, still lightly runny, paste.
Whisk in the vanilla extract, then set aside to cool.
In another small bowl, combine the ground almonds with the bicarbonate of soda and pinch of salt.
Put the sugar, olive oil and eggs into a bowl (if you do not have a free standing mixer) and beat together vigorously for about 3 minutes until you have a pale and thickened yet aerated cream.
Pour in the cocoa mixture, beating as you go, and when all is scraped in you can slowly tip in the ground almond mixture.
Scrape 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.
Let it cool for 10 minutes on a wire rack, still in its tin then remove gently.
Serve warm or cold with sifted icing sugar.