Preheat oven to 180°C (350°F). Place egg whites in a clean,dry bowl of a stand mixer and whisk on high speed until soft peaks form (about 1 minute).
Gradually add sugar, 1 dessert spoon at a time, whisking on medium/high speed until fully dissolved before adding the next spoonful (this will take about 12 minutes, no less).
Mix cornflour and vinegar in a small bowl, then slowly add to the meringue while mixing on low speed.
Using large metal spoons and shape 10-12 quenelles (approximately 10cm in length) on a baking tray lined with parchment paper.
Reduce oven temperature of oven to 110°C (230°F) and bake for 1 hour 10 minutes. Do not open oven. Turn off the oven and allow the meringues to cool completely in the oven overnight with door closed.
Lemon curd:
To prepare lemon curd, in a heat-proof bowl whisk together lemon juice, sugar, butter, and eggs. Place over simmering water, whisking constantly for 2-3 minutes to create foam on top and until thickened.
Remove and whisk for another minute to cool down. Set aside. Lemon curd will last for up to a week.