Why can’t you say’.. envie du chocolat ‘. In your grammas section you do say that ‘de’ should be contracted according to number and gender

Bonjour Nicola,

You can use 'du / de la / des' with 'envie de' if you wish to be specifict. 

Here's an excellent explanation from our Q&A forum about this specific topic: 

envie de / avoir besoin de

I hope this is helpful.

Bonne journée !