Step 1: Prepare the coffee jelly. In a medium saucepan, combine the strong brewed coffee, agar-agar powder, and granulated sugar. Whisk thoroughly to dissolve the agar-agar and sugar.
Step 2: Bring the mixture to a rolling boil over medium-high heat, stirring constantly to prevent sticking. Continue boiling for 1-2 minutes to ensure the agar-agar is fully activated and dissolved. Pro Tip: Boiling is crucial for agar-agar to set properly. Don't rush this step!
Step 3: Carefully pour the hot coffee mixture into a shallow dish or a square baking pan (about 8x8 inches). Let it cool at room temperature for about 15-20 minutes, then transfer it to the refrigerator to chill and set completely, which usually takes 1-2 hours. The Vietnamese Iced Coffee Jelly Dessert needs to be firm.
Step 4: While the jelly is setting, prepare the condensed milk sauce. In a small bowl, whisk together the sweetened condensed milk and whole milk (or half-and-half) until well combined and smooth. Taste and adjust sweetness if desired. Pro Tip: For a richer sauce, use a bit less milk; for a lighter sauce, add a splash more.
Step 5: Once the coffee jelly is firm, remove it from the refrigerator. Using a knife, cut the jelly into small 1/2-inch to 1-inch cubes. You can also use fun-shaped cookie cutters for a playful twist on your Vietnamese Iced Coffee Jelly Dessert.
Step 6: To serve, divide the coffee jelly cubes among 4 serving glasses or bowls. Add a generous amount of ice cubes to each glass.
Step 7: Pour the prepared condensed milk sauce over the coffee jelly and ice. Stir gently before enjoying. Serve immediately for the best experience of this delightful Vietnamese Iced Coffee Jelly Dessert.