Combine 2/3 cup (150 mL) milk and flour in a screw top jar. Shake vigorously until smooth. In a medium saucepan over medium heat, melt butter. Add brown sugar and boil until slightly caramelized, 2 to 2 1/2 minutes. Add 2 cups (500 mL) milk all at once (butterscotch will harden). Cook and stir until butterscotch is dissolved. Stir in flour mixture. Bring to boil, stirring constantly. Cook 2 minutes, stirring constantly. Stir about 1/4 of the hot mixture into egg yolks and mix quickly. Pour egg yolk mixture back into saucepan. Cook and stir over medium heat 1 minute, without boiling. Add vanilla. Pour about half of the mixture into pie shell. Let cool 5 minutes. Add chocolate to remaining filling and stir until well blended. Spoon carefully into pie shell, over butterscotch mixture. Preheat oven to 200 C (400 F). In a medium bowl, beat egg whites with cream of tartar until foamy. Add sugar gradually, about 1 tbsp (15 mL) at a time, and continue beating until sugar is dissolved and meringue is stiff but still glossy. Spoon over warm pie filling, make sure meringue adheres to pie shell edge, covering filling completely. Bake until meringue is golden, about 8 minutes. Cook on a cake rack.
Refrigerate uncovered until ready to serve.
Preparation: 15 minutes
Cooking: 15 minutes