Let the grass jelly slide out of the can onto a cutting board. If it doesn't slide out easily, carefully cut the jelly in half while it's still in the can, then let both pieces slide out. Cut half into ½-inch cubes and the other half into 1-inch cubes. Transfer each size to a separate bowl and set aside.

Add the light brown sugar and cold water to a small saucepan and stir to combine. Bring to a boil over medium heat, then reduce to a simmer. Stir frequently until the sugar dissolves and the syrup thickens enough to lightly coat the back of a spoon.

Divide the warm brown sugar syrup in half and set one portion aside. Divide the other portion between the two bowls of grass jelly. Gently toss each with a spatula until evenly coated.

Divide the grass jelly between 4 tall serving glasses, layering some small and medium cubes below and above 5 ice cubes in each glass.

Let the reserved syrup cool slightly, then divide it between the glasses, slowly spooning it along the inside walls to create caramel-colored streaks.

Slowly pour 1 cup of cold milk into each glass over the grass jelly, ice, and brown sugar syrup. Serve immediately.