Mix cornstarch and sugar. Add 4 tablespoons butter and half and half. Cook over medium heat, stirring constantly, until mixture boils and becomes thick and creamy. Remove from heat and stir in the vanilla.
Preheat oven broiler to high.
Pour mixture into pie crust. Drizzle 2 tablespoons butter over top and sprinkle with cinnamon. Put under broiler until butter bubbles--watch it carefully as it doesn't take long. Refrigerate for at least 1 hour before serving.