CHOCOLATE作可数名词时是有巧克力糖块的意思.所以可以加S.
a box of chocolates /HAVE ANOTHER CHOCOLATE 
但如果作为 
1.事物的名字时 
2.作巧克力饮料时 
3.形容颜色即深褐色、巧克力色时.是不可数的. 
它还是个形容词即用巧克力制成,巧克力色的的意思. 

不可数 
a hard brown sweet food made from COCOA beans, used in cooking to add flavour to cakes, etc. or eaten as a sweet/candy 巧克力；朱古力 
Eg: a bar/piece of chocolate 一条/一块巧克力 a chocolate cake 

可数 
a sweet/candy that is made of or covered with chocolate 巧克力糖；加心巧克力 
Eg: a box of chocolates 一盒巧克力糖