In the situation you sketch, it is often about finding a good balance between formal/informal, eagerness/disinterest, especially when there is a business relationship between you and the other person.
A good approach that works for me is one where there is some surprise, some hesitance, and finally a heartfelt thank you, for example:
Ooh.. Are you sure? Hmm, thank you, I appreciate it!
While rejecting an offer might be seem like the most polite thing to do in some situations, accepting an offer can have its own rewards. For example, accepting/sharing food creates and/or promotes trust, plus there's the Ben Franklin-effect to consider, which might yield its own rewards further down the road.
//updated reply to include OP's later elaboration of the question//
Helping oneself to something would only be socially acceptable if that something was presented in a way that suggests that it's up for grabs. In the situation you initially described, helping yourself to some Lindt bar would make for a socially awkward situation, especially if the other person were still holding that bar. It can be seen as an invasion of privacy and/or a violation of someone's personal space and/or property.
"I can certainly help myself to some Lindt now." when you are offered or even handed a piece is not a contextually sound response.
Don't overthink it though - even if your response is a little off, if it's clear that you are non-native but are trying your best, I can't imagine your response backfiring on you that quickly.