In a small pot, combine 2 tablespoons of the milk, sugar, and cocoa powder. Heat over medium heat, whisking constantly, until mixture begins to simmer and is well-combined and smooth.
Remove from heat and whisk in cold coffee, remaining 2 tablespoons of the milk, and optional heavy cream.
Pour over a 16-ounce cup (or mason jar) of ice, top with whipped cream and chocolate syrup if desired, and enjoy.
*If you don't have cold coffee, you can brew a pot at double strength and pour over a cup of ice (about 1 heaping cup of ice per 1 cup of coffee).
OR pour hot coffee into a large cup and place it in the freezer for 15 to 20 minutes until it has cooled to at least room temperature.
To make a mocha frappuccino, make the recipe as written, using heaping tablespoons of sugar instead of level (the ice will mute the sweetness so you'll want a bit more) and blend 1 1/2 cups ice with the mocha.