Preheat the oven to 200°C. Roll the pastry out to 5mm thickness on a lightly floured surface. Cut into 4 rounds that will line 4 x 6cm individual fluted tart tins. Prick the bases and chill.
Meanwhile, prepare the filling by combining the lemon juice, cornflour and sugar in a small saucepan. Add 100ml of water and bring to a simmer over a moderate heat, whisking continuously until you have a thickened lemon sauce. Once the mixture starts to bubble, remove from the heat and beat in the butter until smooth and glossy.
Whisk together the egg yolks and whole egg in a separate mixing bowl, then add to the lemon filling and return to the heat. Continue to whisk all the time until the mixture thickens until it falls from the whisk when tapped against the side of the saucepan. Remove from the heat and set to one side.
Remove the pastry from the fridge and line with nonstick baking paper and baking beans. Bake blind for 10-12 minutes until the edges just start to colour, then remove from the oven and discard the paper and baking beans. Return to the oven for 4-5 minutes to brown the bases. Remove once browned a little and brush the bases with the beaten egg to create a seal. Leave to cool on a wire rack.
Prepare the meringue by whisking the egg whites to soft peaks in a large, clean mixing bowl. Add of the caster sugar, a tablespoon at a time, beating well between additions. Whisk in the cornflour, then the remaining sugar the same as before, whisking well between additions until you have a thick, glossy meringue. Spoon into a piping bag fitted with a 3cm straight-sided nozzle.
Reheat the lemon filling and spoon evenly between the pastry cases. Pipe dots of the meringue all over the surface of the filling, working quickly. Bake in the oven for 12-15 minutes until the meringue is golden at the tips and cooked. Remove and allow to sit for a few minutes before turning out and serving.