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Pour the water and agar agar into a small saucepan. Whisk until combined, then set the saucepan on your stove over high heat. Bring the mixture to a boil before continuing to the next step. Note that agar agar powder (also called "kanten powder") works best, but you could use three-quarters of an agar stick if necessary. Break the stick into pieces and soak them for 20 minutes in separate water. Drain the soaking water, then use the broken pieces of agar as you would use the powder. Equal amounts of unflavored powdered gelatin could be used instead of the agar agar powder, but gelatin is not vegetarian-friendly. Once the mixture starts to boil, reduce the heat to medium. Pour the sugar and instant coffee into the saucepan and whisk to combine. Continue cooking the mixture at a gentle simmer for approximately 2 minutes, or until everything dissolves. Stir the mixture occasionally to help it along. Turn off the heat and remove the saucepan from the stove. Allow it to rest at room temperature for approximately 5 minutes. During this time, the liquid should begin to thicken. Make sure that it does not solidify, though. Agar sets quickly, and waiting too long can make it difficult to pour. Pour the hot mixture into individual serving dishes. Wait another 5 to 10 minutes, then cover each dish with plastic wrap. After pouring the mixture into each dish, use a spoon to scoop away any bubbles that form on the surface. Place the filled dishes in your refrigerator. Chill them until the jelly sets and becomes thoroughly cooled. Jelly made with agar will technically set even at room temperature, but the process will take more time. Moreover, the jelly will taste better cold. The coffee jelly is finished and ready to enjoy. Consider serving each portion with whipped cream or 1 to 2 Tbsp (15 to 30 ml) of half-and-half. Leftover coffee jelly should last for up to 2 days when stored in the refrigerator.
Heat the water and agar agar. Add the sugar and coffee. Remove the saucepan from the heat. Pour into serving dishes. Chill for 4 to 5 hours. Serve.