½ cup (120ml) calamansi juice (or 6 tablepoons (90ml) lemon juice + 2 tablespoons (30ml) orange juice)
For the meringue:
4 egg whites
¼ tsp (1g) cream of tartar
½ cup (115g) superfine sugar
¼ teaspoon (1g) cornstarch
¼ teaspoon (1g) vanilla
For the pastry, combine flour, sugar and salt in a large bowl. Cut in butter with a pastry cutter or two knives until mixture resembles coarse crumbs. Beat egg and milk together in a small bowl. Add to flour mixture and stir together to form a shaggy dough. Turn onto a lightly floured surface and knead gently to bring together. Wrap in plastic wrap and form into a disk. Refrigerate 1 hour.
Heat oven to 400F (205C).
Roll dough on a lightly floured work surface to ¼-inch thick. Transfer to a 9-inch (23cm) pie plate, trim and crimp edges. Refrigerate for 15 minutes. Dock the bottom all over with a fork. Line with aluminum foil or parchment paper and fill it with pie weights or dried beans.
Bake crust for 20 to 25 minutes, until beginning to set and brown at edges. Remove foil or parchment and weights. Continue to bake until golden, about 8 to 12 minutes. Cool completely.
For the filling, whisk together sugar, cornstarch and salt in a medium saucepan. Slowly whisk in water and turn heat on to medium. Bring mixture to a boil, whisking frequently, until thick and glossy, about 3 to 5 minutes.
Place the yolks in a heat-proof bowl. Slowly stream in about 1 cup (225g) sugar mixture into egg yolks. Add to remaining sugar mixture and cook, whisking constantly, over medium heat for 1 minute. Remove from the heat and whisk in butter, vanilla, zest and juice. Pour into cooled crust. Cover surface of filling with plastic wrap while preparing meringue.
To make the meringue, beat egg whites and cream of tartar on medium speed with a hand mixer in a large bowl until foamy. Whisk together sugar and cornstarch in a small bowl. Increase speed to high and add sugar mixture one tablespoon (15g) at a time until the egg whites hold medium peaks.
Remove plastic from filled pie crust and dollop about half meringue onto filling, spreading to ensuring it touches crust all the way around pie. Using a butter knife or skewer, swirl through the meringue and filling to further affix the two together. Dollop remaining meringue onto the pie and swirl.
Bake until meringue is golden, 5 to 7 minutes. Cool to room temperature. Refrigerate at least 4 hours before serving.