Step 1: Make the Pastry Base

1. Prepare the pastry: In a mixing bowl, combine the flour and icing sugar. Rub in the chilled butter using your fingertips until the mixture resembles fine breadcrumbs. Stir in the egg yolk and cold water, a little at a time, until the dough comes together.
2. Roll out the dough: Turn the dough onto a floured surface and roll it out to fit your pie dish. Carefully line the dish with the pastry, trimming the edges.
3. Chill and bake: Preheat the oven to 190°C (375°F). Chill the pastry in the fridge for 20 minutes, then prick the base with a fork. Line the pastry with baking paper and fill with baking beans or rice. Blind bake for 15 minutes, then remove the paper and beans and bake for another 5 minutes until the base is golden. Set aside to cool.

Step 2: Prepare the Lemon Filling

1. Mix the cornflour: In a small bowl, blend the cornflour with a little water to make a smooth paste.
2. Make the lemon curd: In a saucepan, heat 300ml of water with the lemon zest and juice. Stir in the sugar and bring to a boil. Once boiling, add the cornflour paste and whisk continuously until thickened.
3. Add the egg yolks and butter: Remove the pan from the heat. Stir in the egg yolks and butter until smooth and glossy. Pour the lemon filling into the pre-baked pastry case and spread evenly.

Step 3: Make the Meringue

1. Beat the egg whites: In a clean bowl, whisk the egg whites until soft peaks form. Gradually add the caster sugar, a tablespoon at a time, while whisking continuously until the meringue is thick and glossy.
2. Top the pie: Spoon the meringue onto the lemon filling, spreading it to the edges of the pie. Use the back of a spoon to create swirls or peaks for a decorative finish.

Step 4: Bake the Pie

1. Bake to perfection: Lower the oven temperature to 150°C (300°F) and bake the pie for 20-25 minutes, or until the meringue is golden and crisp on the outside.
2. Cool before serving: Allow the pie to cool slightly before slicing. Serve warm or at room temperature.