Preheat the oven to 140C/275F/Gas 1 and butter a 30cm/12in x 20cm/8in cake tin.
For the parkin, put the soft brown sugar, butter, syrup and treacle into a small saucepan and melt over a gentle heat.
Put the oats, flour, ginger, nutmeg, mixed spice and a pinch of salt into a large bowl, then add the eggs and milk. Pour the melted butter and sugar mixture into the bowl and mix together with a whisk until combined.
Pour into the prepared tin and bake for 1¼ hours, or until firm in the centre.
Remove from the oven and leave in the tin to cool before turning out and cutting into squares. This is best kept kept in a tin for 2-3 days as it will become really sticky all the way through.
For the rhubarb, put the butter, sugar and five tablespoons of water in a large frying pan with the orange zest and rhubarb. Place over a gentle heat to poach for 4-5 minutes, or until just tender, then remove from the heat.
For the syrup, combine the golden syrup, cider and spices in a saucepan and bring to the boil. Reduce the heat and cook for 3-4 minutes, or until just thickened.
Serve the parkin in squares with a spoonful of rhubarb to one side and the syrup over the top. Finish with a scoop of ice cream.