Hot chocolate is a drink made from cocoa powder or (in the case of luxury versions) from melted chocolate. It was served on the Knight Bus. While Harry Potter was on the bus in 1993 he ended up spilling his hot chocolate before he was able to drink it.
Molly Weasley served this on the evening of 31 August in 1992, following Fred and George Weasley giving an exhibition of Dr. Filibuster's Fabulous Wet-Start, No-Heat Fireworks. The drink is also made at Hogwarts, as Albus Dumbledore was going downstairs to get a cup one night early 1992, which led him to discover a Petrified Colin Creevey. Later that year, Dumbledore suggested that Ginny Weasley's should have some, after her encounter in the Chamber of Secrets, saying "I always find that cheers me up."