In a medium microwave-safe bowl, mix together cornstarch, sugar, and salt. Whisk in milk. Microwave uncovered in 2 minute increments, stirring after each increment. Continue until mixture is thickened, about 6 to 7 minutes.
Add vanilla extract, chocolate chips, peanut butter, and marshmallows. Stir until mixture is smooth. Pour into pie crust. Chill at least 3 hours before serving.