1
: Place sweetened condensed milk, milk, cocoa and cornstarch in a medium saucepan. Stir to combine well.
2
: Cook mixture, stirring frequently, on medium heat until slightly thickened, about 8-10 minutes. Remove from heat, stir in chocolate chips and vanilla until melted. Strain mixture into bowl.
3
: Wrap with plastic wrap and chill in refrigerator 1 hour. Stir whipped topping into cold mixture. Top with additional whipped topping if desired. Serve.