To make the meringues, whisk the egg whites in a large bowl until stiff then gradually whisk in the caster sugar, a teaspoonful at a time. Whisk for a minute or two more until the meringue is very thick and glossy.
Mix the cornflour, vinegar and vanilla together in a small dish then whisk into the meringue. Spoon the mixture into 6 mounds on a large baking sheet lined with non-stick baking paper and swirl into circles with the back of the spoon. Bake in a preheated oven, 110°C (225°F), Gas Mark ½, for 50–60 minutes or until the meringues can be easily peeled off the paper. Leave to cool on the baking tray.
Meanwhile, mix the pineapple in a bowl with the chopped mint and caster sugar. Cover and chill until required.
When ready to serve, whip the cream until softly peaking. Stir the pineapple and fold half the fruit and any juices into the cream. Transfer the pavlovas to serving plates, spoon the cream on top then spoon over the remaining pineapple. Decorate with mint leaves, if liked, and serve within 30 minutes.