Mix sugar, cocoa, cornstarch & salt in medium pan. Combine eggs & milk. Slowly stir into sugar mixture, till well combined. Cook over medium heat, stirring constantly w/wisk till mixture starts to thicken and just comes to a boil. Remove from heat.
Add choc chips, butter & vanilla. Stir till everything is melted smooth. Pour into crust.
Cover w/plastic wrap, making sure wrap is directly on the pie surface, so skin won't form. Chill in fridge 4-5 hours till firm. Top w/whipped cream & choc chips.