Combine softened butter and flour in a mixing bowl until it resembles coarse crumbs. Add confectioners' sugar and the room temperature egg, mixing until a dough forms. Shape into a ball, wrap in cling film, and refrigerate for 30 minutes.

Preheat oven to 180°C (356°F). Roll out the chilled pastry to 3mm thick, line a flan tin with it, trimming excess. Cover with parchment and add weights, then blind bake for 15 minutes. Remove weights and bake for an additional 5 minutes until pale golden.

Whisk fresh lemon zest, lemon juice, and cornstarch in a saucepan. Gradually stir in boiling water and cook on medium heat, stirring until thick, about 2-3 minutes. Temper egg yolks by adding some hot mixture to them, then incorporate back into the saucepan and heat until creamy.

Whip egg whites in a clean bowl until soft peaks form, about 3-4 minutes. Gradually add superfine sugar, beating until stiff peaks develop and the mixture is glossy.

Pour lemon filling into the pre-baked pastry shell, smoothing evenly. Let cool for 10-15 minutes. Spread meringue over filling, touching the edges to seal. Create peaks and bake at 170°C (340°F) for about 15 minutes until golden brown.

Cool to room temperature before slicing and serving.