Have your crust ready and begin prep for your filling by mixing together the sugar, cornstarch and salt in a medium saucepan. Whisk in the water. Set it aside while you make the meringue.
Meringue: In a small saucepan, whisk the cold water and cornstarch together in a small saucepan. Heat over medium, stirring constantly, until mixture forms a thick gel; remove from heat.
In the bowl of stand mixer, starting at low speed and gradually increasing, beat the egg whites and cream of tartar until soft peaks form. When soft peaks form, beat in the marshmallow cream in 3 parts. Mixture should look smooth and start forming stiff peaks. Beat in 3 tablespoons of gel and then add the vanilla. At this point you should have stiff peaks. Set it aside for a few minutes while you make the filling.
Set your saucepan with the cornstarch mixer over medium heat. Whisk constantly over medium heat until the mixture starts thicken and become translucent. Whisk in a small portion of the egg yolks, then whisk in the rest. Add the lemon juice and zest and whisk another minute or just until smooth and thick and well mixed.
Remove from heat and add the butter, whisking until it’s melted. Pour hot filling into the baked pie crust.
Spread the meringue over your hot filled pie and bake at 375 for about 12 minutes – covering top loosely with foil after the first 5 minutes if needed. Let cool at room temperature for about an hour, then transfer to the refrigerator and chill for about 4 hours.