Preheat the oven to 350F. Mix the graham cracker crumbs, sugar, and melted butter, then press firmly into a 9-inch pie dish. Bake for 8-10 minutes and let cool.
In a saucepan, whisk together the sugar, cornstarch, and salt. Gradually whisk in the water and lemon juice until completely smooth.
Cook over medium heat, whisking constantly, for 5-7 minutes until the mixture is very thick and bubbling. Cook for 1 more minute to fully activate the cornstarch.
Remove from the heat and stir in the butter and lemon zest. Pour into the cooled crust, let cool to room temperature, then refrigerate for 4 hours until set.
For the meringue, beat the aquafaba and cream of tartar on high speed for about 5 minutes, until soft peaks form.
Add the sugar one tablespoon at a time while beating, then continue whipping until stiff, glossy peaks form - about 8-10 minutes total. Beat in the vanilla.
Pile the meringue over the chilled pie, spreading it to touch the crust all around and creating swirls with the back of a spoon.
Toast the meringue with a kitchen torch until golden, or place under the broiler for 1-2 minutes, watching constantly. Serve the same day for the best texture.
