Boil 2 cups of water in a saucepan. Once boiling, remove from heat and add the black tea leaves or tea bags. Let steep for 5 minutes.
Strain the tea into a bowl or pitcher, discarding the leaves or bags.
While the tea is still warm, whisk in the cocoa powder and sugar until fully dissolved.
Allow the mixture to cool to room temperature, then refrigerate for at least 30 minutes.
Once chilled, combine the cocoa tea with the milk in a glass filled with ice cubes. Stir well to combine.
Garnish with chocolate shavings or a sprinkle of cocoa powder if desired. Serve immediately.