Preheat your oven to 140°C (fan) or 160°C conventional. Line a baking tin with parchment paper.
In a saucepan over medium heat, add the butter, black treacle, and milk. Stir occasionally until the butter has melted and everything is fully combined. Remove from heat and set aside to cool slightly.
In a large mixing bowl, add the self-raising flour, oats, dark muscovado sugar, and ground ginger. Mix together thoroughly.
Pour the melted treacle mixture into the dry ingredients. Add the beaten egg.
Mix everything together with a fork (or spoon) until fully combined. The mixture will be quite thick.
Pour the batter into your lined baking tin and smooth the top.
Bake for approximately 1 hour, or until the centre feels just firm to the touch.
Allow the parkin to cool in the tin for 15 minutes before transferring to a wire rack to cool completely.
For traditional texture, wrap the cooled parkin in parchment paper and store for a few days before slicing. This allows moisture to redistribute and the cake to become properly sticky.