Combine the apple cider, brown sugar, cinnamon, nutmeg, and fine salt in a small saucepan. Whisk until the sugar begins dissolving.

Place the saucepan over medium heat and bring the mixture to a gentle simmer. Reduce the heat to medium-low and cook for 7–10 minutes, stirring occasionally. The syrup should reduce slightly and smell strongly of apple and cinnamon. It will remain fairly fluid while hot and should not become as thick as pancake syrup.

Remove the saucepan from the heat: Stir in the vanilla extract. Transfer the syrup to a heatproof measuring cup or jar.

Let the apple spice syrup cool for about 30 minutes, or until it reaches room temperature. For an especially cold drink, refrigerate the syrup after it has stopped steaming. Do not pour hot syrup directly over ice.

Divide the cooled apple spice syrup evenly between two 16-ounce glasses. Add 1 tablespoon (20 g) caramel sauce to each glass. Stir until the caramel is mostly incorporated into the syrup. If your caramel is especially thick, mixing it with the syrup before adding the cold coffee will help it dissolve.

Pour 1 cup (240 mL) chilled coffee into each glass. Stir thoroughly until the apple syrup and caramel are evenly distributed. The mixture should look uniform without a heavy layer of caramel sitting at the bottom.

Pour ¼ cup (60 mL) whole milk into each glass. Stir gently. Taste before adding the ice. The coffee should be slightly stronger and sweeter than your ideal finished drink because the ice will dilute it somewhat.

Add about 1 cup (140 g) ice cubes to each glass. Stir once more until the drink is thoroughly chilled.

If desired, top each iced coffee with whipped cream. Drizzle with the optional caramel sauce and finish with a very light dusting of cinnamon. Serve immediately.