Preheat oven to 180C. Place unpeeled plum halves, skin side up, in a baking dish. Add the 150ml of water and rosewater and sprinkle with the 60g of sugar. Roast for 15 minutes until tender. Remove from oven and separate plum halves from liquid, which should be pink. Place liquid in a small saucepan and bring to boil, reducing by three-quarters. In a bowl, whisk yolks and 50g of sugar until mixture is pale and thick. Add the 100ml of water and grappa and mix well. Place bowl over a pot sitting on a medium heat, filled to a third with simmering water. The bowl should sit snugly on the pot but the bottom of the bowl should not be touching the water. Keep whisking the zabaglione until it thickens and doubles in volume. To serve, place three plum halves in each serving plate, pour a little of the reduced cooking liquid in, then the zabaglione, with the pistachios on top.