In a medium saucepan, whisk together cocoa powder, sugar, salt, and cornstarch (if using).
Slowly whisk in 1 cup milk until completely smooth.
Cook over medium-low heat for 2–3 minutes, whisking constantly, until smooth and the sugar dissolves. Do not boil.
Remove from heat and stir in vanilla extract. Whisk in 1 more cup cold milk to cool slightly.
Pour into a large pitcher with remaining 6 cups milk. Whisk, blend, or shake very well until combined.
Chill until cold. Shake before serving.
Store covered in the refrigerator until the milk’s expiration date. Shake before each use.
