Whisk 90ml grapefruit juice, zest, sugar, salt, and eggs in a pan over gentle heat.
Sift in cornflour and whisk continuously until the mixture is thick.
Remove from heat and whisk in the butter one cube at a time.
Cover curd with clingfilm touching the surface and refrigerate for at least one hour.
Preheat oven to 180°C and butter six 8x5cm metal dessert rings.
Cream butter, sugar, salt, and lemon zest until pale and fluffy.
Add egg yolks and beat into the creamed mixture.
Sift flour and baking powder into the mixture and beat into a smooth paste.
Pipe dough into rings in a 4mm thick spiral and add a raised outer ring.
Bake for 12 to 15 minutes or until golden.
Cool for two minutes, loosen with a knife, and transfer to a wire rack.
Heat sugar and 40ml water over high heat to 118°C.
Whisk egg whites and salt in a bowl until a light froth forms.
Pour hot sugar syrup into egg whites in a thin stream while whisking.
Add pepper and whisk for 10 minutes until glossy and stiff peaks form.
Top biscuits with grapefruit curd and meringue.
Brown the meringue using a blowtorch or hot grill.