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.