Step 1

Heat the oven to 180°C/fan oven 160°C/mark 4, then butter and line the base of a 20cm spring-form cake tin. Break the chocolate into pieces and place in a heatproof bowl set over a pan of simmering water. Stir a little to help the chocolate to melt.

Step 2

Once it's completely melted, whisk in the oil in a steady stream, then two-thirds of the sugar, whisking to help the sugar dissolve in the heat of the chocolate. Remove from the heat. Stir in the ground nuts, a pinch of flaked sea salt and the egg yolks.

Step 3

Put the egg whites into a scrupulously clean bowl with about one third of the remaining sugar. Beat with an electric whisk until the whites are no longer clear, and then add another third of the sugar. Continue beating until the whites have really increased in volume, then add the rest of the sugar and beat until you have medium peaks (firm with tips that droop slightly).

Step 4

Using a really large metal spoon, loosen the chocolate mixture by folding in a big tablespoon of the egg whites, then fold in the rest carefully so that you don't lose air. Scrape the batter into the prepared tin and bake for 40 minutes. Test by inserting a fine skewer into the centre of the cake. If it comes out clean, with no batter attached, the cake is ready.

Step 5

Leave it to cool in the tin - it will deflate and crack a lot, but that is fine. Carefully turn it out and remove the paper. Put on to a plate and dust with icing sugar before serving. This is a chic, grown-up cake - it needs nothing more than some crème fraiche on the side.