In a medium sauce pan combine cocoa powder, sugar and constarch. Add in milk and vanilla. Bring to a boil over medium heat. Let boil 1 minute.
Remove from heat and add butter.
Pour into prepared crust and cool 3 hours or overnight.
Top with marshmallows. Place pie on the lowest rack in the oven and turn the broil on. Broil just until marshmallows brown.
