Preheat the oven to Gas 4, 180°C, fan 160°C.
Bake the potatoes in the oven on for an hour until cooked. Remove from oven and leave to cool enough so they can be handled.
Increase the oven temperature to Gas 6, 200°C, fan 180°C. Slice potatoes into ½ inch slices. Melt the butter and marmite together in the oven, in a metal bowl ideally, making sure to stir regularly so that it isn't too sticky.
Dip each potato slice into the marmite mixture, ensuring they are thoroughly covered.
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