For the meringues, whisk the egg whites and lemon juice in a clean bowl until soft peaks form when the whisk is removed.
Add the sugar a third at a time, whisking after each addition, until stiff peaks form when the whisk is removed.
Bring the milk and the vanilla bean paste to the boil in a large, shallow, lidded pan. Reduce the heat until the mixture is just simmering.
Dip 2 tablespoons in hot water, then make 8 quenelles of the meringue mixture, using the spoons to shape the meringue. Carefully lower each quenelle of meringue into the simmering milk. Cover the pan with the lid and poach for 7-10 minutes, carefully turning the meringues over halfway through cooking.
Remove the poached meringues from the pan using a slotted spoon and set aside to drain on a small tray until needed. Reserve the poaching milk and keep hot.
For the vanilla cream, lightly whisk the egg yolks and caster sugar together in a mixing bowl, until smooth.
Gradually add the hot milk to the egg yolks, whisking continuously, then return the custard mixture to the pan and heat over a medium heat, stirring continuously with a wooden spoon, until the custard is thick enough to coat the back of the spoon.
Pour the custard immediately into a large serving bowl and suspend it over a bowl of ice cubes or chilled water. Stir occasionally until the custard cools.
When the custard has cooled, transfer to a serving bowl. Float the poached meringues on top of the cooled custard, then chill in the fridge for up to 24 hours.
For the caramel, pour the water into a straight-sided pan, then add the sugar and cook over medium heat, until the sugar has dissolved. Once no sugar crystals remain, boil the mixture until caramelised to a dark golden-brown.
Carefully dip the base of the pan into a bowl or sink full of cold water to prevent the caramel from cooking any further.
Pour the caramel over the floating meringues and serve immediately.