Parkin is pretty much a northern English thing. Some versions are made with golden syrup and are quite light, but the Yorkshire version of parkin uses black treacle (molasses) and oats and has quite an earthy flavour. Try to make it a few days ahead of when you intend to eat it because it really does improve with age.
If you have time, leave the batter in the fridge overnight, or for a few hours at least. This will help the oats absorb some of the moisture.
Traditionally, parkin is made as a loaf, but you can do it as a traybake if you like, and cut it into squares. Line a loaf tin or roasting tin with greaseproof paper or a silicone mat and pour the batter in - for a traybake it should be about 3-4cm (about an inch and a half) deep. For a loaf the batter should come almost to the top. Bake the parkin for 60-75 minutes. You can check whether it's done by inserting a skewer into the centre - if it comes out dry the parkin is done. Tip it out onto a wire rack to cool. If you're making a traybake, cut off the crusty edges and cut the rest into squares.
You can substitute corn syrup for golden syrup, and molasses for black treacle.