Preheat the oven to 120C/Gas ½ and line a baking sheet with greaseproof paper. Clean your chosen whisking bowl with a drop of lemon juice, to degrease it.
Whisk the whites in an upright mixer or with an electric hand-whisk on top speed. When they start to go frothy, add the sugar a tablespoon at a time, whisking constantly. Once the mix has thickened and all the sugar is in, stir the vinegar and cornflour together in an eggcup and tip this in too. Carry on whisking at full pelt for about another 3 minutes until firm and glossy
As soon as the mix is ready, dollop on to the prepared tray. For individual meringues, use a large serving spoon or spatula to make 4 Willy Wonka-esque constructions or 6–8 kiddysized ones. For a pavlova, dollop the whole lot into the middle and spread out to a circle roughly 25cm across, with a higher rim around the edge and a lower, flatter plateau in the middle.
. Bake for 1¼ hours, without opening the oven at any stage, then turn the oven off and keep the door shut. Leave the meringue/s in there until the oven has cooled completely. When meringue munching time is near, whip the cream with the caster sugar until thick but still a touch floppy. Use it as extravagant glue to stick two meringues together and watch with joy as small jaws try their hardest to wrap themselves around the prize.