Place the egg whites in an electric mixer bowl and whisk on medium speed until soft peaks form. Gradually add the caster sugar, in 3 batches, followed by the cornflour and vinegar. Turn up the speed to full for a few seconds to completely incorporate the egg whites and then turn off the machine.
Bring the milk and vanilla to 78°C (172°F) in a heavy-based wide-based saucepan. Whisk the m