In a 3 quart saucepan, combine 2 cups of the milk, sugar, cocoa and butter. Bring the mixture just to a boil over medium heat.
While the mixture in the saucepan is heating up, in a separte bowl, mix cornstarch and cold water until completely dissolved. Stir in remaining 1/2 cup of milk, then whisk or stir in the egg yolks and vanilla until well blended.
Gradually whisk the egg mixture into the chocolate mixture and continue cooking over medium heat, stirring constantly, for about 2 minutes or until mixture is thickened and smooth. Add marshmallows and remove from heat. Stir until marshmallows are melted and mixture is smooth.
Pour into pie shell. Press plastic wrap directly onto filling and let cool at room temperature for about 30 minutes. Transfer to the refrigerator and let chill until set (at least 2 hours).
Spread sweetened whipped cream over the top of the pie. You can use homemade sweetened whipped cream or Cool Whip.