250g glacé chestnuts or marrons glacés
75ml dark rum
50g caster sugar
400g good-quality chocolate cake (at least 23cm diameter x 2cm deep)
375g dark chocolate, roughly chopped
200g sweetened chestnut purée
500ml double cream
1 tbsp cocoa powder
2 tbsp icing sugar
Roughly chop about two-thirds of the glacé chestnuts. Place the rum and caster sugar into a saucepan and bring to a simmer. Cook until the sugar has dissolved. Remove from the heat and add the chopped glacé chestnuts. Stir well, then leave to cool.
Line a 25cm cake tin with a double layer of cling film. Allow plenty of overhang. Place the chocolate cake in the base of the tin, then scatter the soaked chestnuts and rum over.
Place the chocolate into a bowl set over a pan of boiling water and leave to melt. Remove from the heat and whisk in the sweetened chestnut purée. Cool slightly. Pour the cream into the chocolate and whisk to combine. Pour the mixture into the cake tin to cover chestnuts and cake. Place in the fridge for at least 2 hrs, preferably overnight.
Carefully lift the tart out onto a plate and gently pull away the cling film. Mix the cocoa and icing sugar together, then use to dust the tart. Decorate with the remaining glacé chestnuts.