Whittaker's said they would be stopping the production of the product in question while they investigate what happened.
"We believe this is an isolated incident, but have taken the precautionary measure of halting the production line of this product, and immediately began investigating how it could have happened.
"The customer was very understanding, and we will of course be sending them a replacement block along with some of our other products they are a fan of.
"We have very robust, best practice quality control processes in place, and it is extremely rare for instances like this to occur.
"We are taking the situation very seriously as is reflected by the immediate actions we have taken."
The chocolate was part of a new line of artisan flavours released by Whittaker's recently.