I remember reading about this in a general travel book (I think it was Rick Steves)! The book recommended buying fresh barnacles in Santiago's outdoor market and having one of the nearby market restaurants preparing them!
The market was easy to find in Santiago, but I did not find barnacles the day I was there! Instead, I ordered them fresh (& prepared) at a restaurant on the water in Finisterre on my last day! So excited! They were delicious!