Combine the cornstarch and water and then place over medium high heat stirring constantly until it has thickened into a gel (about two minutes) and set aside.
In a large glass bowl, lightly beat the egg whites and cream of tartar to dissolve. Begin beating on medium-low speed until foamy and begin slowly sprinkling the sugar in while beating, giving the sugar time to dissolve. Continue to beat on medium speed until stiff glossy peaks form.
Then add the cornstarch gel and beat to combine. Sprinkle either the hot pie, room temperature, or chilled pie with donut crumbs, if using, and then apply a large dollop of meringue. Spread the meringue to the edges of the pie and seal the pie completely by pressing the meringue against the crust. Add the remaining meringue and swirl with a spatula so it pleases your own personal aesthetics.
If preparing a hot filling meringue, immediately top with the pre-prepared meringue and brown. If opting for a cold filling, allow the filling to cool completely and then transfer to the refrigerator. Set the oven to ‘broil’ and set a rack near the bottom of the oven.
Broil the pie meringue until golden brown. Do not overcook, as this will cause beading on the top of the pie.
For the hot filling method, place the pie on a wire rack and allow it to cool to room temperature before serving. For the chilled filling pie, allow the pie to stand for 15 minutes before cutting with a clean damp knife and serving.