Melt the chocolate and the butter together in a pan over gentle heat. In another pan, heat the chestnuts with the milk until just boiling, then mash thoroughly with a potato masher (or process in a blender).
Separate the eggs, put the yolks in a bowl and mix with the caster sugar. Stir in the chocolate mixture and the chestnuts until you have a smooth, blended batter. Whisk the egg whites until stiff and fold them carefully into the batter.
Transfer the mixture to a greased, lined 25cm cake tin (springform) and bake at 170C (gas mark 3) for 25 to 30 minutes, until it is just set.
Leave to cool a little, then release the tin, and slice carefully.
