Heat the oven to 200°C/180°C fan/gas 6. Place the sweet potatoes on a baking tray then transfer to the oven to roast for 45 minutes or until tender. Remove from the oven and leave to cool.
Once cool enough to handle, scoop the potato flesh into a medium bowl and mash with the mixed spice and 2 tbsp water. Transfer to a medium saucepan and place over a medium/low heat. Cook on a steady simmer for 15 minutes, stirring often, until thickened. Reduce the heat if it starts to stick. Take off the heat and beat in the butter, then spread on the toast or rice cakes.