Preheat the oven to 180°C/350°F (without fan).

Using a standmixer or electric beaters, whisk the egg whites in a large, clean bowl until they have tripled in volume and soft peaks form.

Turn the speed down to low and slowly add the caster sugar, one spoon at a time until the sugar is well incorporated. As you add the sugar, the mixture will start to become thick and glossy, and stiff peaks will form when you lift the beaters.

Remember to scrape down the sides of the bowl.

Rub a bit of the mixture between your fingertips to test if the sugar has been fully dissolved; if you don't feel any grains of sugar, move onto the next step.

Add the cornflour, white wine vinegar, vanilla extract and cocoa powder. I do this on low speed with the stand mixer for only 2 to 3 rotations (about 1 second).

Fold through the chopped chocolate.

Line a baking tray with baking paper and spoon the pavlova mixture onto the tray into a round shape. I like to dollop the mixture on top of each other into a high pile, and to then shape the pavlova from there - using this method ensures that your pavlova will have some height.

Use a palette knife to shape the sides of the pavlova and to slightly flatten the top. The pavlova will expand and rise once cooked, so shape it to be on the slightly smaller side. 

Place the pavlova in the oven and immediately turn the temperature down to 150°C/300°F (without fan).

Bake for 1 hour and 15 minutes. Do not open the oven door during this time.

After 1 hour and 15 minutes, turn off the oven and leave the oven door closed so the pavlova can cool down completely. You should leave the pavlova in the oven for at least 2-3 hours, but overnight (12-18 hours) is best.

Place the chestnuts, milk, vanilla extract and rum into a medium saucepan.

Simmer gently until the chestnuts have softened and can be mashed easily with a wooden spoon, and most of the milk has been absorbed.

Take the pan off the heat and mash the chestnuts with a potato masher until you have a smooth purée - you might need to add some more milk if the mixture is too thick.

Return the pan to the heat, stir through the sugar, and cook for a few minutes until the mixture binds together. Set aside to cool.

Just before serving, whip the cream until soft peaks form.

Place a generous layer of the chestnut purée on top of the pavlova. To achieve the characteristic noodle strands, you will need to pipe the mixture using a spaghetti or grass nozzle (some countries sell a nozzle specifically for chestnut purée), or you can achieve a similar effect by pushing the chestnut purée through a potato ricer. If the chestnut purée is too thick or too firm, stir through some milk to soften the mixture. If you don’t have a special nozzle or potato ricer, simply spread the chestnut purée over the pavlova using a palette knife.

Dollop the whipped cream on top of the chestnut purée, and grate over some chocolate to decorate.