Prepare the pie crust by rolling it out and placing it into a pie dish. Trim and crimp the edges, then dock the bottom with a fork. Freeze briefly, then bake with weights until set and lightly golden. Set aside to cool.
Combine sugar and cornstarch in a saucepan, then whisk in water and egg yolks until smooth. Cook over medium heat while stirring constantly until the mixture thickens and comes to a boil. Reduce heat and continue cooking until fully thickened.
Remove from heat and stir in butter until melted. Strain the mixture into a bowl, then mix in lemon zest and lemon juice. Pour the filling into the prepared crust and chill until fully set.
For the meringue topping, place sugar, egg whites, cream of tartar, and salt in a heatproof bowl over simmering water. Whisk until the sugar dissolves and the mixture is warm. Transfer to a mixer and beat until stiff peaks form, adding vanilla during mixing.
Spread the meringue over the chilled pie, ensuring it touches the crust edges. Create decorative swirls, then toast the top with a torch or under a broiler until golden. Serve immediately or chill briefly before serving.
