Place rack in lower third oven and preheat to 350 º. For filling, place egg yolks in small bowl. Combine sugar, cornstarch, and salt in heavy saucepan. Whisk in Lemon Chiffon Soy Smoothie, and zest. Set pot over medium heat, and bring liquid to a simmer. Cook, whisking constantly, until steam rises from mixture, 3-5 minutes. Pour one-quarter of hot liquid into egg yolks, whisking vigorously so eggs do not cook. Whisk this mixture back into pot. Add lemon juice. Cook over medium-low heat, stirring constantly, until mixture is thick enough to coat a wooden spoon and does not run at the edges of a line drawn with your fingertip down back of spoon, 2-3 minutes in the pot. Take care not to let mixture boil, or eggs will curdle. Immediately pour filling into a bowl. Whisk in butter until it melts. Set filling aside to cool slightly.
For meringue, place egg whites in an immaculately clean bowl. Using electric mixer and freshly washed beaters, beat whites on slow speed until they are foamy. Beat cream of tartar and salt into the white. Increasing speed to high, beat until egg whites form soft peaks. One tablespoon at a time, beat in sugar, mixing for 15 seconds before adding the next tablespoon. Continue beating until meringue is thick, glossy and forms stiff peaks, up to 5 minutes. It should resemble marshmallow fluff.
Spread warm filling in baked pie shell. Heap meringue on top of filling. Using a rubber spatula, spread meringue out to the edge of the crust, keeping it piled highest in center of pie. Make sure meringue touches the fluted edge of the crust, sealing filling underneath. If desired, use back of a soup spoon to swirl and lift meringue, making peaks.
Bake until meringue is golden brown, with dark brown peaks, about 15 minutes. Cool pie on a rack to room temperature before serving.
This pie is best made the day it will be served, to avoid egg whites ?weeping? where pie filling and topping meet. This pie can keep up to 2 days in refrigerator. To store, cover pie, tented loosely with foil coated with cooking spray on side facing meringue to avoid sticking.