A deep-fried Mars bar is an ordinary Mars bar normally fried in a type of batter

Batter (cooking)

Batter is a semi-liquid mixture of one or more flours combined with liquids such as water, milk or eggs used to prepare various foods. Often a leavening agent such as baking powder is included to aerate and fluff up the batter as it cooks, or the mixture may be naturally fermented for this purpose...

 commonly used for deep-frying

Deep frying

Deep frying is a cooking method in which food is submerged in hot oil or fat. This is normally performed with a deep fryer or chip pan; industrially, a pressure fryer or vacuum fryer may be used....

 fish, sausages, and other battered products. The chocolate bar is typically chilled before battering to prevent it from melting into the frying fat, though a cold Mars can fracture when heated.

The dish originated at chip shops in Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 as a novelty item, but was never mainstream.