Cocoa, also known as hot chocolate, was a beverage made from chocolate.
In Mexico in 1454, the First Doctor drank cocoa with Cameca, which in Aztec culture meant that the two of them were engaged to be married. (TV: "The Bride of Sacrifice") Some time later, Tlotoxl and Tonila prepared cocoa in honour of vanquishing Barbara Wright, who was posing as Yetaxa. (HOMEVID: Making Cocoa)
Evelyn Smythe loved cocoa so much that she insisted upon bringing her own supply of cocoa mix on her initial trip in the TARDIS. She shared some with William Leaf and George Crow, but Crow didn't like it, preferring ale. (AUDIO: The Marian Conspiracy) After she witnessed the death of Cassie Schofield, the Sixth Doctor offered Evelyn some chocolate cake and cocoa to help her feel better; it did not help. (AUDIO: Project: Lazarus)
The Tenth Doctor and Donna Noble enjoyed some hot chocolate together inside the TARDIS on Christmas Eve. Donna declared it was her "favourite thing in the world". (PROSE: Dr. Tenth: Christmas Surprise!)
Behind the scenes
Dez Skinn drinks a cup of cocoa (comically labelled as such, as though this needed pointing out) as he goes to bed in the comic story The Origin of Doctor Who Magazine, printed in DWM 544 to celebrate the 40th anniversary of Doctor Who Magazine. However, due to its parodical nature, this story is not currently considered a valid source for in-universe articles by our Wiki.