Hot chocolate (or hot cocoa) was a beverage made with chocolate, and served heated.
Ensign Sonya Gomez accidentally spilled hot chocolate on Captain Jean-Luc Picard near the beginning of her tour on the USS Enterprise-D. (TNG episode: "Q Who?")
Following this incident, Gomez was given the nickname "Ensign Hot Chocolate" (later shortened to "HC") by her crewmates. (SCE eBook: Many Splendors)
While traveling on the shuttlecraft Jefferies, Doctor Beverly Crusher and Deanna Troi enjoyed two cups of hot chocolate while traveling back to the USS Enterprise-E. (TNG novel: A Time to Harvest)