In separate bowl, whisk sugar, arrowroot powder, and remaining 1/2 cup cold whey until it forms a smooth paste
Add paste mixture into the hot whey, stirring constantly until it thickens
In another small bowl, lightly beat egg yolks and combine with butter, salt, and lemon juice
Temper by pouring a small amount of the hot mixture into egg yolk mixture and stirring, to ensure you don’t end up with bits of scrambled eggs
Add tempered mixture back into saucepan, cook entire concoction an additional two minutes, stirring constantly
Pour filling into prebaked pie shell
Set aside and prepare meringue
Prepare Meringue:
Use electric mixer to beat egg whites, cream of tartar, salt, and vanilla extract in a bowl until entire mixture is foamy, but not yet starting to thicken
Begin to add sugar, one tablespoon at a time
Beat thoroughly after each addition
Dissolve sugar completely to prevent “weeping” droplets from forming on your meringue
Once all sugar is added, continue to beat mixture at medium-high speed until it forms stiff, sharp peaks when you lift mixer from bowl
Place large spoonfuls of meringue around the edges of pie and use back of spoon to gently “seal” it to crust to help keep it from shrinking
Once you have a ring of meringue around the outside, dump remaining meringue in a mound in center and smooth out evenly
Use back of spoon to make little spikes like you see in my photos
Carefully place pie in preheated oven and bake at 350 degrees 12-15 minutes