Preheat your oven to 350°F (175°C) and roll out your pie crust into a 9-inch pie dish. Bake until golden brown for about 10-15 minutes. Set aside to cool.

In a medium saucepan, melt 6 tablespoons of butter over medium heat. Stir in 1 cup of brown sugar until dissolved, about 2 minutes.

Add 4 egg yolks, 2 cups of milk, and 2 tablespoons of cornstarch, whisking until smooth. Cook while stirring continuously until thickened, around 5-7 minutes, then remove from heat and stir in 1 teaspoon of vanilla extract.

In a clean mixing bowl, beat 4 egg whites until frothy. Gradually add 1 cup of sugar and 1/4 teaspoon of cream of tartar, beating until stiff peaks form, about 5-7 minutes.

Pour the butterscotch filling into the cooled pie crust and spread it evenly. Dollop the meringue on top and seal the edges.

Bake in the preheated oven for 10-15 minutes until the meringue is golden brown. Let cool for at least 2 hours before slicing.