I've made soft treacle parkin before but I think I prefer this recipe from the BBC. The gingerbread gets more flavoursome after a few days.

1 egg
4 Tbs milk (I use rice)
200g butter
200g golden syrup (I use a mix of golden syrup and rice malt syrup
85g treacle
85g brown sugar
100g oatmeal (or oats coarsely ground)
250g self-raising flour (I use spelt and 1 tsp baking powder)
1 Tbs ground ginger

Preheat oven to 160C. Line a 20cm square cake tin with baking paper.
Beat the egg with the milk. In a saucepan melt the butter, syrup, treacle and sugar. Remove from the heat. Mix the oats, flour and ginger in a bowl. Stir in the syrup mixture followed by the egg mixture.
Pour the batter into the cake tin and bake for 45min-1 hour until the cake feels firm and there's a crust on top. Cool in the tin then wrap in baking paper and foil. The parkin will become softer and stickier the longer you leave it, and will keep for up to 2 weeks.