In a thick-bottomed saucepan, whisk together the Nesquik powder and cornstarch.
Slowly whisk in the milk until the mixture is completely smooth.
Place the pan over medium heat and whisk constantly.
Continue cooking until the mixture thickens to a heavy pudding consistency.
Remove from heat and immediately stir in the vanilla extract.
Pour the mixture into a pre-baked pie shell or individual serving bowls.
Serve warm as a pudding, or refrigerate until set for a firm pie.
Top with Cool Whip if desired before serving.