Line an 8-inch (20 cm) tart or pie pan with the pastry crust.
Line the pastry with wax paper and fill the crust with dried beans or rice.
Bake in a preheated 400F (200C) oven for 10 minutes.
Remove the paper and beans and bake an additional 5 to 10 minutes, until the crust is light golden brown.
Remove the crust from the oven and set aside.
Meanwhile, combine the brown sugar and cornstarch in a mixing bowl.
Bring the milk to a boil in a saucepan and pour over the brown sugar mixture, stirring to dissolve the sugar.
Return the mixture to the saucepan and bring to a boil over moderate heat, stirring constantly, and continue to stir while the mixture boils gently for 3 minutes.
Remove from the heat and stir in the butter, vanilla, and egg yolks.
Stir over low heat for an additional 2 minutes, and pour the mixture into the pie shell.
Beat the egg whites until stiff but not dry, adding the sugar and cream of tartar a little at a time as you beat.
Mound the egg whites on top of the butterscotch filling and bake in a preheated 300F (150C) oven until the meringue is lightly browned, about 30 minutes.
Serve warm, chilled, or at room temperature.