For the meringue, heat oven to 140C/fan 120C/gas 1. Line the base of a loose-bottom 20cm square tin. Tip 25g of the walnuts into a food processor and blitz until ground as finely as possible, add the rest
of the walnuts, then pulse to chop roughly. Tip the egg whites into a clean bowl, whisk until peaks have formed, then gradually add the sugar a third at a time, beating between each addition until stiff. Fold in the walnuts. Tip the meringue into the
tin, then spread over the base. Bake for 2 hrs until crisp, then leave
to cool. This can be done 1 day in advance and covered in cling film.
While the meringue is cooking, make the sweetened purée by mixing the chestnut purée with the sugar until smooth. This can be done 1 day in advance and covered in cling film. Put a couple of large bowls to chill
in the fridge.
To assemble, leave the ice cream
to soften slightly so it’s pliable, but not melted. Working quickly, in one of the bowls mix half of the ice cream with half of the purée to make chestnut ice cream. If it’s too soft once you have mixed it, place the bowl in the freezer to firm up. Once all the ice cream is the same consistency, quickly ripple the vanilla and chestnut ice creams and half the remaining purée together. Spread over the meringue and smooth the top, then place in the freezer until frozen solid. This can be done 1 month in advance and frozen, covered in cling film. You can also freeze the rest of the purée.
To serve, defrost the purée, remove the cake from the freezer, then place
in the fridge for 40 mins. Meanwhile, make a thin piping bag from greaseproof paper and whip the cream until stiff. Mix in the rest of
the purée, continue to beat until
it becomes stiff again, then tip the cream into the piping bag. Using
a large knife dipped into hot water, cut the cake into 9 squares. Working quickly, pipe a ‘crazy-string’ nest of chestnut cream in the centre of each square. Top the cream with half a marron glacé or candied chestnut
and serve straight away.