In a large quart-sized jar or other airtight container, combine the coarsely ground coffee, dark brown sugar, and ground cinnamon.

Pour the water over the coffee mixture and stir thoroughly until the brown sugar has dissolved and the cinnamon is evenly distributed throughout the liquid.

Cover the jar tightly with a lid and place it in the refrigerator. Allow the mixture to cold brew overnight, or for at least 8 to 12 hours, so the coffee develops a smooth, full-bodied flavor while the cinnamon and brown sugar gently infuse every sip.

The next day, strain the brewed coffee through a fine-mesh sieve, coffee filter, or fine strainer into a medium bowl to remove the coffee grounds. For an even cleaner brew, strain it a second time before transferring the finished coffee back into the storage jar.

To prepare a serving, fill a tall glass about halfway with ice cubes. Pour the cold coffee over the ice until the glass is approximately three-quarters full.

Finish by adding milk or half-and-half to your preferred level of creaminess. Stir gently to combine and adjust the amount of dairy to suit your taste.

Serve immediately and enjoy a refreshingly smooth iced coffee with subtle sweetness and warm cinnamon notes. For a personalized twist, try adding a drop of almond extract with a pinch of crushed fennel seeds, substitute cardamom for the cinnamon, or replace the milk with sweetened condensed milk for a richer, creamier variation.