Heat oven to 140C/120C fan/gas 1. Line a large, flat baking tray with baking parchment. Place the egg whites in a large, grease-free bowl with a pinch of salt. Using clean, electric beaters, whisk until the eggs hold soft peaks. Add the sugar, 1 tbsp at a time, whisking well between each addition, until the meringue is thick and glossy, and will hold up in stiff peaks on the end of the beaters. Add the vinegar and cornflour and whisk again until combined. Using a large spoon, pile the meringue onto your baking tray, then spread to a rectangle, roughly A4 size. Use the back of the spoon to make peaks and dips in the meringue to create an attractive texture. Bake in the centre of the oven for 1 hr 20 mins, then turn off the oven and leave the Pavlova inside to cool overnight.