Directions:
Mix Dry Ingredients:
In a medium saucepan, whisk together the brown sugar, cornstarch, and salt.

Heat the Mixture:
Gradually add the milk to the saucepan while whisking continuously. Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and starts to bubble (about 5-7 minutes).

Temper the Eggs:
In a small bowl, whisk the egg yolks. Slowly add about 1/2 cup of the hot mixture into the yolks while whisking to prevent curdling. Then, return the egg mixture to the saucepan and cook for an additional 2 minutes, stirring constantly.

Finish the Filling:
Remove the pan from heat and stir in the butter and vanilla extract until fully melted and incorporated.

Fill the Pie Crust:
Pour the warm butterscotch filling into the pre-baked pie crust. Smooth the top with a spatula.

Chill:
Let the pie cool to room temperature before covering it with plastic wrap and chilling in the refrigerator for at least 4 hours or overnight, until fully set.