Heat the oven to 140 C /275 F/ 120 Fan/ Gas 1.
Grease a 20cm square baking tin.
Put the butter, syrup and treacle into a pan and heat until just melted, then remove from the heat.
Beat the egg and the milk together in a cup. Mix the oatmeal, flour, ginger, nutmeg, sugar and salt together in a bowl, pour in the melted treacle mixture and mix together, then beat in the egg and milk.
Pour into the prepared tin. Bake in the oven for 75 minutes, or until springy in the centre. Cool in the tin then cut into squares.
You can also leave the Parkin in the tin, unsliced, and store for up to three weeks – it will get more gooey as time goes on.