Grandma's Butterscotch Pie

In top of double boiler, combine brown sugar, cornstarch, salt and milk.

Cook, stirring constantly, until mixture starts to thicken, about 20 minutes.

Whisk in egg yolks; continue to cook and stir until filling is thickened.

While pudding mixture is cooking, preheat the oven to 400 degrees F (200 degrees C).

Remove filling from heat, and stir in butter and vanilla.

Pour filling into prepared pie crust.

Bake in preheated oven until top begins to brown, about 5 minutes.