Carefully pull the skin on the chicken legs away from the flesh, trying not to break the skin, then rub ¼ tbsp butter between the skin and flesh on each leg. Carefully put half of the truffle discs on top of the butter, or smear on some truffle paste and smooth back the skin. Season the skin, then set aside in the fridge.
Heat the double cream and stock together with the garlic cloves, remaining truffle, bay and sage then, once simmering, set aside to infuse for 30 mins.
Heat oven to 180C/160C fan/gas 4 and butter the inside of a shallow baking dish that will fit both chicken legs. Layer the potatoes, adding the truffle slices from the cream between the layers along with some seasoning, then strain over the infused cream and stock mixture. Sprinkle over the parseman and scatter over the remaining sage leaves. Place the chicken legs on top, brush with melted butter, then roast for 1 hr or until the potatoes are cooked and the chicken golden and cooked through. Allow to rest for 10 mins, letting the chicken juices drip into the potatoes, then serve with a watercress salad.