For the grapefruit curd: Measure 6 tablespoons/90 milliliters of grapefruit juice into a pan and whisk together with the zest, sugar, salt and eggs over a gentle heat. Sift in the cornflour and continue to whisk. Don't stop whisking at any point, otherwise the eggs will curdle.
Once the curd is as thick as pureed tomatoes and has released a bubble or two, take it off the heat and whisk in the butter a cube at a time. Pour into a bowl and cover with cling film, placing it in direct contact with the curd to prevent a skin forming. Refrigerate for at least an hour (best overnight).
For the biscuit base: Preheat the oven to 350 degrees F/180 degrees C/Gas 4 and butter six 3- by 2-inch/8- by 5-centimeter metal dessert rings.
Cream together the butter and sugar with the salt and lemon zest until fluffy and pale in colour. Add the egg yolks and continue to beat. Sift the flour and baking powder together, add to the creamed mixture and continue beating until the dough comes together as a smooth paste.
Put the dough into a piping bag fitted with a 1/3-inch/1-centimeter round nozzle. Pipe the dough into the rings in a spiral working from the outside towards the centre until the base is completely covered by a 1/8-inch/3- to 4-millimeter layer, and then pipe a ring round the edge to create a little dip for the curd to sit in. Bake until golden (but not too dark), 12 to 15 minutes.
Remove the biscuit base from the oven and leave to cool for a couple of minutes before running a small sharp knife around the inside of each ring to release the biscuit. Transfer the biscuits to a wire rack (be careful as they're fragile) and leave to cool.
For the Italian meringue: Put the sugar into a pan with 1 fluid ounce/40 milliliters water and place on a high heat. Bring to the soft-ball stage (245 degrees F/118 degrees C on a sugar thermometer) which will take about 10 minutes. To test without a thermometer, drop a tiny bit of sugar syrup into a bowl of very cold water. When it forms a soft sticky ball, it is ready.
While waiting for the sugar syrup, start whisking the egg whites with the salt in a clean glass or metal bowl. Whisk until a light froth forms, stopping before any soft peaks can be formed. Once the sugar syrup has reached the soft-ball stage, beat the egg whites at the same time as pouring the syrup onto them in a thin stream. (Don't pour the syrup over the whisk, but down the side of the bowl.) Add the pepper and continue to whisk for 10 minutes or until the egg whites are glossy and stiff.
To assemble, place a generous tablespoon of the grapefruit curd on top of the biscuits, followed by the meringue. Either place under a very hot grill for a couple of minutes or use a blowtorch to brown.