Preheat oven to 140° C fan. On a piece of greaseproof paper, draw a 23cm circle with a black pencil (use the base of a 23cm baking pan as guide). Turn the paper over. Set on a baking tray until needed.
Time to whip the egg whites. Make sure your egg whites are at room temperature and your mixing bowl and beaters are clean and grease free (see note). Put egg whites in the bowl and beat on high speed until foamy. While whisking, start adding the sugar, one tablespoon at a time. Add the cornflour, cream of tartar, vinegar and vanilla. Keep beating until you have a glossy and stiff meringue. Don’t over-beat.
While the meringue is still in the mixer bowl, drizzle with the melted chocolate but DON’T MIX! Take large dollops of the meringue and mount in the centre of the circle you drew on the greaseproof paper. Using your drawing as a guide, spread meringue to a rough 23cm circle. While you spread the meringue the melted chocolate will leave swirls of unmixed chocolate. Build up the sides of the meringue slightly, leaving a dip in the middle. That’s where your chestnut cream will go.
Transfer meringue in the oven and bake for one hour. Switch off the oven and leave the oven door ajar. Use a wooden spoon to keep the door slightly open. Let meringue cool completely in the oven at least an hour longer. Sometimes I leave it over night.
Take the tray out of the oven and set aside the meringue (still on the paper) until needed. The chocolate should be completely set before you try to take the meringue off the paper. If it cracks don’t sweat. That’s the beauty of the meringue. It’s so forgiving. Nobody will notice.
Make the chestnut cream: Put cooked chestnuts, milk and sugar in a small pot. Bring to boil and reduce heat to medium. Let chestnuts simmer, 15 minutes or so, until milk is almost evaporated and mixture looks creamy. Transfer mixture in a blender or food processor and blend on high speed until pureed and smooth (some chestnut bits are ok). Transfer in a bowl and add the chopped chocolate. Mix with a spatula. The chocolate will eventually melt. If it gives you a hard time, heat the mixture over low heat, stirring constantly until you have a smooth chocolatey mixture. Cool cream to room temperature.
Beat 1 cup cream until soft peaks form. Fold cream in the chestnut mixture. Mix until you have a smooth, mousse-like cream. Go on taste it. It’s divine!
Assemble the pavlova: Carefully peel the paper off the meringue. Transfer meringue on a serving platter. Spread the chestnut cream in the centre leaving a small border.
Whip the 1 ½ cups cream with the icing sugar until stiff peaks. Mount cream on top of chestnut cream. Garnish with shaved chocolate, caramelised nuts or more melted chocolate if you wish. You can make meringue up to o 5 hours in advance. Keep refrigerated.