Make the crust: Whisk flour, granulated sugar, and salt in a bowl. Using a dough blender, two forks, or your fingers, cut butter into flour mixture, forming pea-size crumbles. Add water; work dough until smooth but with visible flecks of butter. (Alternatively, pulse ingredients in a food processor.) Flatten dough into a disk and wrap in plastic wrap; chill 1 hour.
On a lightly floured surface, roll dough into a 12" round. Fit into a 9" pie plate. Trim edges and crimp; chill 30 minutes.
Heat oven to 375°. Using a fork, prick dough all over. Line dough with parchment paper and fill with pie weights or dried beans; bake until golden, about 20 minutes. Remove paper and weights; let cool.
Make the filling: Combine granulated sugar, cornstarch, salt, and 1 1⁄2 cups water in a 2-qt. saucepan. Cook, whisking, over medium-low heat for 3 minutes, then remove from heat. Transfer 2 tbsp. sugar mixture to a bowl; add yolks and whisk until smooth. Whisk yolk mixture into remaining sugar mixture and bring to a simmer over medium-low heat. Cook, whisking constantly, until thickened, about 3 minutes. Whisk in butter, lemon zest, and juice; let cool.
Make the meringue: Heat oven broiler. Using an electric hand mixer, beat egg whites in a bowl until soft peaks form. With the motor running, gradually add cream of tartar and superfine sugar; beat until stiff peaks form. Spread lemon filling evenly over crust; spread meringue evenly over filling, piling it slightly higher in the center. Using a spatula, create peaks all over meringue. Broil, rotating pie as needed, until meringue is browned in spots, 1–2 minutes. Cool completely before serving.