Preheat the oven to 180˚C. Grease a deep 20 cm cake tin and line the base with baking paper.
Simmer the chestnuts in the milk for 15 minutes then drain and let it cool.
Whizz the chocolate in a food processor until you get a coarse mixture. Set aside.
Drain the chestnuts then put them in the food processor together with the hazelnuts and whizz to a rough paste. Seet aside.
Put the butter and sugar in a large bowl and beat them until creamy. Blend in the egg yolks, one at a time. Mix in the brandy and orange zest the fold in the chocolate and the chestnut mixture.
In another bowl whisk the egg whites until you get stiff peaks. Fold it gently into the other mixture, first a couple of spoonful, followed by the rest.
Transfer the batter into the cake tin, making sure the surface is levelled.
Bake for 30 minutes until it’s risen nicely and properly cooked through.
Cool the cake for 10 minutes in the tin, then carefully turn it out onto a wire rack.
When the cake is completely cool, dust it with some icing sugar.
Serve it with custard, ice-cream, chocolate sauce, crème fraîche etc.