Weigh the cocoa powder into a large heatproof bowl. Add 150ml boiling water from the kettle, and stir until smooth. Leave to cool.

Heat the oven to 180C/160C fan/gas 4, with a shelf in the centre of the oven. Oil 2 x 20cm loose-bottomed cake tins and line the bases with baking parchment. Add the yogurt, milk, vinegar, eggs, soy sauce, olive oil and 1/4 tsp fine salt to the bloomed cocoa powder, then whisk well until there are no lumps.

Combine the flour, baking powder, bicarb and sugar in a separate large bowl. Shake the bowl a few times to bring any lumps of sugar to the surface and squeeze these between your fingers until the mixture is an even, sandy consistency. Pour in the wet ingredients and combine using an electric whisk until the batter is smooth and loose.

Divide the batter between the tins and bake in the centre of the oven for 25 mins until a skewer inserted into the centres come out clean. If any wet mixture clings to the skewer, bake for another 5 mins, then check again. Leave the cakes to cool completely in their tins. Can be made up to two days ahead and kept well wrapped at room temperature.

For the ganache, put the chopped chocolate in a heatproof bowl. Warm the cream in a small pan over a low heat until steaming, or do this in the microwave. Pour the warm cream over the chocolate and stir until smooth and glossy. Leave to cool while you make the mascarpone filling (or until just spreadable, if you’re skipping the mascarpone. This will take between 20-40 mins, depending on the temperature. If the ganache is too loose, it will drip off; too firm and it'll be too hard to spread. Stir regularly to ensure it has reached the right consistency). Tip the mascarpone, cream, icing sugar and vanilla into a bowl and whisk to soft peaks.

Invert one of the cooled sponges on a serving plate or cake stand, and peel off the parchment. Spoon the mascarpone cream over the centre and spread out to the edges, trying not to disturb the cake crumbs too much. Sandwich the other sponge on top. (Alternatively, sandwich with a quarter of the ganache.)

Cover the top and sides of the cake with ganache using a palette knife, then sprinkle over a few sea salt flakes, if you like. Leave to rest for about 1 hr until the ganache has firmed up slightly, or chill for 30 mins before serving. Will keep chilled for up to two days (or, if you've made a ganache-only cake, keep in an airtight container at room temperature for up to four days). Remove from the fridge 1-2 hrs before serving.