Disclaimer: when in doubt about what you can eat and when you can eat it, always go by your surgeon's/dietician's instructions, they have degrees and stuff plus get paid to know what is best for you.
Yes, I was allowed hot chocolate on the liquid diet post-op. I used options (low cal) but have heard here that highlights is better. A tip to increase the nutritional value of hot choc directly post-op, add powdered skim milk to the hot choc powder and then add hot water/milk. This will increase the protein value & calories which are essential early days.
HTH,
Nic