In large pot off the heat, measure sugar, cocoa powder, corn starch, and salt. Whisk together to combine. Slowly whisk in cold milk, using a spatula where necessary to reach the corners, until all the milk is added.
Place the pot over medium-high heat and whisk continuously until mixture thickens and just starts to boil. Remove from heat immediately and stir in vanilla, butter and chocolate chips. Stir until chocolate has melted completely and the mixture is smooth.
Pour into baked pie shell, smooth the top and cover the top with plastic wrap directly so the filling is touching the plastic. Refrigerate 6-8 hours or overnight.
Pie will be ready once plastic wrap removes cleanly off of filling. Spread crushed OREOS over the top of the pie with any additional candy decorations. Serve.