The Carron Fish Bar in Stonehaven is usually credited with frying the first one in the early 1990s, and visitors still travel there specifically to try it.
A whole Mars bar is chilled, dipped in the same batter used for haddock, and deep-fried until the chocolate inside turns molten and the batter crisps up gold. It's sold in paper straight from the fryer at chip shops up and down Scotland's east coast, usually with a fork stuck in the top.