It depends where you are in the world. In Australia we have the same Mars Bar as the UK, which is caramel and nougat coated in milk chocolate. In the US, a Milky Way is the equivalent of our Mars Bar, with their Mars Bar being more similar to a Snickers Bar, as they both contain nuts. Moro bars are also a good substitute, but really, any of these bars are equally suitable to use in this brownie recipe as they are all very similar products.