Preheat oven to 200C/180C fan/gas 6.
Make pastry: Pulse flour, butter, icing sugar in food processor. Add yolks, water, pulse to form ball. Chill.
Roll out pastry, line tin, prick base, line with paper, fill with beans, bake 15 mins, remove beans, bake 5-10 mins until golden. Cool.
Reduce oven to 180C/160C fan/gas 4.
Make lemon filling: Mix cornflour, caster sugar in saucepan. Gradually stir in lemon juice, zest, 450ml water. Cook until thickened, boil, simmer 1 min. Remove from heat, stir in butter, yolks until smooth. Pour into pastry case.
Make meringue topping: Whisk egg whites, salt until stiff. Gradually whisk in sugar, cornflour, lemon juice. Spoon onto filling, spread, pipe swirls.
Bake 15-20 mins until golden, set. Cool before serving.