If you look very closely, you may manage to see steam rising from the surface of the coffee - yes, it is actual steam, not a Photoshop fake as I originally intended!
It was achieved with a hand held steamer in one hand, and the camera remote in the other, with a bedside light suspended nearby to throw extra light on the cup and its contents. I processed with Silver Efex because it highlighted the contrast more than Colour Efex. The biggest problem I had was trying to gauge the right amount of steam (plus where to position the steamer) and the right moment to press the remote. Very many of these attempts looked like the cup was coming out of a sauna, or its contents contained witches' brew rather than anything you would want to drink.
This is great! Good experiment.