Instructions

1. Preheat oven to 180°C.
2. Melt the chocolate and butter in a bowl set over boiling water, making sure the base of the bowl is not touching the water. When chocolate has almost completely melted, remove the bowl from heat and stir the mixture until smooth.
3. In a large mixing bowl, whisk the egg yolks with half the sugar and the salt until you get a pale and thick cream. Stir in the warm chocolate, then set aside.
4. In another bowl, whisk the egg whites with the cream of tartar until fluffy, then add sugar and beat until stiff (but not dry).
5. Add the chestnut flour and chopped chestnuts to the chocolate mixture, then gently fold in some of the egg whites. Combine gently, then gradually fold in the remaining whites. You should get a nice, light batter.
6. Transfer the batter into a medium spring-form pan, smoothing the top.
7. Bake for 25 to 30 minutes, until a skewer inserted in the centre comes out almost clean.
8. Turn off the oven, open its door and let the cake cool there for 5-10 minutes, then move it to a wire rack. Let it cool, then carefully release the spring.
9. Serve warm with your chosen sauce or let it cool completely, then dust with icing sugar before slicing.