In 2-quart saucepan, mix brown sugar, cocoa and cornstarch with whisk. Gradually stir in milk. Cook over medium heat 5 to 7 minutes, stirring constantly, until pudding thickens and boils. Boil and continue to stir constantly 1 minute longer. Remove from heat; stir in chocolate chips, chocolate hazelnut butter, butter and 1 teaspoon vanilla until blended. Pour into shallow dish; cool 10 minutes. Press plastic wrap on filling to prevent tough layer from forming on top. Refrigerate 30 minutes or until serving time.