Choc-ice appears to be a bar of chocolate ice cream. The term usually refers to a bar of ice cream covered in chocolate. They can be bought from a local vendor named Rokuh in Nardah, where they are sold for 30 coins each; Rokuh has a supply of ten. When eaten, they heal 300 life points, and quench a player's thirst if they are in the desert. This can come in handy as an alternative to bringing waterskins to the desert, although most players prefer to use waterskins to save inventory space.
Choc-ices were developed to aid the people of Nardah. When the town was cursed and could no longer provide water, Rokuh bought a box of magical non-melting ice. He used this to help keep the people of the dying town hydrated. The magic can last forever, although it will melt in the desert due to the extreme heat.
In the quest Do No Evil a piece of choc-ice in the shape of a monkey, chimp ice, is made for King Awowogei of Ape Atoll. Due to its peculiar shape the chimp ice cannot contain the freezing-inducing magic normally applied to choc-ice and will thus melt away within a few seconds. The player will frequently have to cast an Ancient Magicksice spell on it (they can't use Ice Rush, as it will not freeze the ice).
In your inventory, it's referred to as a "Choc-ice" but when you eat it, it's called a "Choc ice".
Non-melting chocolate ice cream is possibly a reference to 'Charlie and the Chocolate Factory', wherein Grandpa Joe tells Charlie that Willy Wonka has "invented a way of making chocolate ice cream so that it stays cold for hours and hours without being in the refrigerator? You can even leave it lying in the sun all morning and it won't go runny!"