Preheat the oven to 140°C and butter a 30cm x 20cm cake tin.
Melt brown sugar, butter, syrup, and treacle in a saucepan over gentle heat.
Whisk oats, flour, spices, salt, eggs, milk, and the melted mixture in a large bowl.
Pour batter into the tin and bake for 75 minutes until firm.
Cool the parkin in the tin before cutting into squares.
Poach rhubarb, butter, sugar, orange zest, and 75ml water in a pan for 5 minutes.
Boil golden syrup, cider, and spices in a saucepan for 4 minutes until thickened.
Serve parkin squares with rhubarb, warm syrup, and a scoop of ice cream.