For the meringues:
For the grapefruit curd:
Preheat the oven to 110°C (225°F), Gas mark 1/4 (or use a fan oven at 100°C for this).
Line two baking trays with baking parchment. Place the egg whites in the bowl of an electric food mixer with the whisk attachment (or use a handheld electric beater). Whisk at full speed for a few minutes until the egg whites hold fluffy, stiff peaks when the whisk is lifted.
Continue whisking and add the caster sugar one tablespoon at a time, but with a few seconds between each addition. Once all the caster sugar is added in, stop the whisk. Sift in one-third of the icing sugar and fold in carefully, using a spatula, then fold in the second third and finally the last third and the vanilla extract. The mixture should be fluffy and light.
Scoop a heaped tablespoonful of the mixture onto a baking tray, using a second tablespoon to help ease it off, to form blobs (or if you prefer, make a little dip or well in the center to hold the whipped cream and curd). Repeat with all the mixture, place in the oven and cook for 1 1/2 hours until the meringues are crisp on the outside and will lift off the baking parchment.
Place the butter in a saucepan on a medium-low heat and when melted, whisk in the juice, zest, sugar, eggs and egg yolk. Turn the heat down to low and cook, stirring constantly, until thick enough to coat the back of a spoon. Then remove from the heat and allow to cool.
Softly whisk the cream, then place generous blobs on the meringues and drizzle with the grapefruit curd.