Preheat oven to 375°F (190°C). To make pastry, whisk flour with icing sugar and salt. Toss in cold butter until lightly coated with flour mixture. Using pastry-blender or fingertips, cut or rub butter through flour mixture until it resembles breadcrumbs. In another bowl, whisk egg yolks with 2 tsp (10 mL) ice water and vanilla. Drizzle onto flour mixture and toss with fork to form a crumbly "dough". Squeeze some in your hand. The dough should hold together. If not, sprinkle with 1 to 2 tsp (5 to 10 mL) more ice water. Scatter pieces of dough in bottom of 9-in. (23-cm) pie pan. Press dough evenly in bottom and up sides of pan. Freeze 15 to 20 min., or until firm. Line pastry with foil and fill with pie weights. Bake in lower third of oven, 20 to 25 min. or until foil comes away from dough easily. Remove foil and weights. Continue baking another 10 to 15 min. until pastry is golden. Cool completely.
Meanwhile, to make filling, whisk sugar, cornstarch and salt in saucepan. Stir in 1 1/2 cups (375 mL) water, lemon zest and juice. Cook over medium heat, stirring, until mixture boils. Remove from heat. In a bowl, whisk egg yolks. Gradually whisk about 1/4 of the hot lemon mixture into yolks. Then gradually mix egg mixture into saucepan mixture. Return to medium heat and, stirring constantly, cook 2 min. or until mixture is thick (do not allow it to boil.) Remove from heat. Stir in butter until melted. Pour mixture into blind-baked pie crust.
To make meringue, preheat oven to 400°F (200°C). Beat egg whites in large bowl with hand-mixer at low speed until they start to foam. Increase speed to high and add salt and cream of tartar. Gradually beat in sugar. Continue to beat at high speed until stiff peaks form.
Using back of spoon or spatula, spread meringue over filling, touching meringue to pie crust edges all around to prevent shrinking. Swirl to make attractive top. Bake 8 to 10 min., or until meringue is golden. Cool completely on rack in a draft-free area until filling is set. Serve same day as baked.
Do not make meringue on hot humid days as it will not hold well.