For the meringue, preheat the oven to 100C/80C Fan/Gas ½. Line a large baking sheet with silicone or baking paper.
Suspend a heatproof bowl over a pan of barely simmering water, making sure the base of the bowl does not touch the surface of the water. Add the egg whites and sugar to the bowl and start whisking them immediately using an electric whisk, until the sugar has dissolved and the mixture has reached about 60C/140F.
Transfer the mixture to a freestanding food mixer. Continue to whisk until smooth and glossy but still slightly warm, about 2-3 minutes.
Spoon the meringue mixture onto the prepared baking tray, making a large disc with raised edges. Bake in the oven for 2½ hours, or until firm, then remove from the oven and set aside to cool.
In a bowl, whisk the double cream with the chestnut purée and vanilla seeds until soft peaks form when the whisk is removed. Chill, covered, until needed.
When ready to serve, spoon the cream over the cooled meringue, then decorate the top with the marrons glacés. Grate over the plain chocolate and garnish with the fresh chestnut.