This cake has been following me for a long time, but this variation is a new addition. It's very easy to make, but as usual, make sure to use incredibly good chocolate, dark, dairy free.
Break the chocolate into pieces and place in a heatproof bowl set over a pan of simmering water (bain-marie). Stir occasionally to help the chocolate to melt and avoid lumps on the side. Once it's completely melted, whisk in the oil in a steady stream. Then add two-thirds of the sugar, keep whisking: this will help the sugar dissolve in the heat of the chocolate. Remove from the heat.
Stir in the ground hazelnuts, check the temperature of the chocolate is lukewarm and add the egg yolks (you don't want to scramble the eggs!).
Place the egg whites into a clean bowl with about one third of the remaining sugar. Beat with an electric whisk until the whites are no longer clear, 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). 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. Leave it to cool in the tin: don't worry if it deflate and crack a lot, but that's the beauty of this cake. Crispy on the top, moisty and luscious on the inside. Put on to a plate and dust with icing sugar before serving. Enjoy!