VisaHQ lists various items that may be imported free of duty, some that are prohibited and some that are restricted. In the last category is:
• Animals, plants and products thereof including foodstuff is subject to quarantine and may not be allowed to enter. For more information contact nearest embassy or consulate or appropriate authority –plant or animal for required certificates and prohibited items.
So, since Korea itself does not seem to be at all forthcoming about what is and what is not allowed, it seems you are expected to contact nearest embassy or consulate or appropriate authority. Personally I'd not bother. I'd rather take the (I believe infinitesimal risk) of the tea and chocolate being confiscated and then just manage without or, as mentioned, replace it in Korea at a higher price.
There should be no issue with duty, as regardless of personal import allowances there is a de minimis 150,000 KRW (about $130) below which no duty is levied anyway.
I would keep the tea in its original packaging, but more for preserving what little flavour it (Yellow Label!) has than for the sake of Customs. If say the only space I had free was a coat pocket I'd settle for Ziploc bags.
In an answer to Would I be allowed to get on a plane from the UK and fly to America with tea? (where agricultural control is quite strict) a Comment (to an answer by @Dewi Morgan - sadly not to one by Zac Lipton, since Lipton is a famous tea brand) includes:
When entering the US I declare my tea and get a tired smile from the Customs bods