Brew and cool your coffee: Make a strong batch—about double strength.
Let it cool to room temp or chill it. You’ll need roughly 3/4 to 1 cup per serving.
Make a quick chocolate syrup: In a small bowl or mug, stir together 1 to 1.5 tablespoons unsweetened cocoa powder, 1 to 2 tablespoons sugar, and 1 tablespoon hot water. Mix until smooth and glossy.
Add a tiny pinch of salt and a few drops of vanilla if you like.
Shake with milk: Add 1/2 cup milk to the chocolate syrup and stir until fully combined. For extra froth, pour this mixture into a jar with a tight lid and shake for 10–15 seconds.
Combine with coffee: Fill a tall glass with ice. Pour in the cooled coffee, then add the chocolate-milk mixture.
Stir well to blend.
Taste and adjust: Add more milk for creaminess, more coffee for strength, or a splash of simple syrup if you want it sweeter.
Finish and serve: Top with whipped cream and a dusting of cocoa or chocolate shavings if you’re feeling fancy. Sip immediately.