A chocolate bar is a food item that heals 3 HP when consumed. It is used as an ingredient in Cooking but can also be ground to chocolate dust using a pestle and mortar or a knife, which is required for the Plague City quest.
The chocolate bar can be added to a cake with 50 Cooking to create a chocolate cake.
You can find a chocolate bar spawning in the Cooking Guild, Tree Gnome Stronghold, Zanaris, or in The Culinaromancer's Chest during and after the Recipe for Disaster quest, however the chest has low supply which respawns slowly. You can also buy it from Funch's Fine Groceries located in the The Grand Tree, for 12 gp each. You can buy chocolate bars from each of the bakers' stalls in Ardougne market place for 12 gp; their stocks are 7 and 9. You can buy 1 chocolate bar from the food store in Port Sarim for 10gp each. You can buy 25 chocolate bars from Rok's Chocs Box in Nardah for 10gp each.