Chocolate syrup in a black IPA? Sounds like an interesting recipe...
It also sounds like you may have scorched some of that syrup - which is why most brewing instructions will suggest that you remove your kettle from your heat source while adding any syrup or extract, and don't return it to the heat until that syrup or extract is fully dissolved - the stuff tends to settle towards the bottom, scorch, and become caked on like what you're seeing.
On metal kettles, a brief treatment of oxyclean or bar keeper's friend will usually knock that down pretty quickly. I'm not entirely sure what the correct method would be to remove it from an enamel pot, but I'm sure it can be done without having to toss the pot.