Preheat oven to 375 degrees F.
Have your crust ready and begin prep for your filling. Make this meringue first so you’ll have it ready to put on a hot filling.
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 bowl of stand mixer, starting at a low speed and gradually increasing (this should help break up the cream of tartar lumps), 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 (for me, that was about all of it) and then beat in the vanilla. At this point you should have still peaks. Set it aside for a few minutes while you make the filling.
Spread the meringue over your hot filled pie and bake at 375 for 12-15 minutes. Cool at room temperature then chill for 4 hours or until pie is cold and set.