- Place espresso and rum in a small pot.
- Place the sucrose and agar in a small bowl and mix until even. Add to the small pot and blend with a hand blender until even.
- Place the pot over medium high heat and cook until the solution starts to boil. When the solution comes to a boil, reduce the heat to a simmer and set a timer for 2 minutes.
- After the solution has simmered for 2 minutes, transfer it to a quarter sheet pan, transfer it to the refrigerator, and prop it to stay level. Chill until completely set.
- When the coffee cubes are set, cut in 1 cm cubes and store in a deli in the refrigerator.