Preheat your oven to 350°F (175°C). If using a pre-made pie crust, place it in a pie dish and prick the bottom. Bake for 10-12 minutes until lightly golden. Cool completely on a wire rack.

In a medium saucepan, combine 1 cup of brown sugar, 4 tablespoons of unsalted butter, and 2 cups of milk over medium heat. Stir continuously until melted and smooth, about 5 minutes. Whisk 3 egg yolks in a separate bowl and temper them gradually with the hot mixture. Return to the saucepan and cook until thickened, about 5 more minutes. Remove from heat and add 1 teaspoon of vanilla extract.

Pour the butterscotch filling into the cooled pie crust, spreading it evenly.

In a large mixing bowl, beat 4 egg whites on medium speed until soft peaks form, about 2-3 minutes. Gradually add 1 cup of granulated sugar and 1/4 teaspoon of cream of tartar while beating until stiff peaks form, about 5-7 minutes.

Spoon the meringue over the butterscotch filling, sealing the edges to prevent shrinking. Create decorative peaks or swirls on top.

Bake the pie in the preheated oven for 10-15 minutes until the meringue is golden brown. Let cool for at least one hour.

Allow your pie to cool completely at room temperature before serving.