Remove one can of firm black grass jelly from the container and place it on a cutting board. Slice the jelly block in half, then cut one half into small cubes and the other half into medium cubes. Place the two sizes into separate bowls and set aside.

Add the brown sugar and cold water to a small saucepan. Bring the mixture to a boil over medium heat, then reduce to a simmer and stir consistently until the sugar dissolves and the syrup thickens.

Pour half of the warm brown sugar syrup evenly between the two bowls of grass jelly cubes. Gently toss the small and medium cubes with a spatula until each piece is coated, then reserve the remaining syrup for creating the layered glass effect.

Add a mixture of the syrup-coated small and medium grass jelly cubes to the bottom of a tall serving glass. Place five ice cubes on top of the jelly layer to create a chilled base.

Add another layer of grass jelly cubes over the ice. Slowly spoon the reserved brown sugar syrup along the inside walls of the glass, allowing it to create visible caramel-colored streaks.