Preheat the oven to 180°C. Wash the potatoes under cold running water. Place on a tray in the oven and bake until tender for 70–90 minutes, depending on their size. When cooked, remove the potatoes from the oven and allow to cool. Cut the potatoes in half, scoop out the flesh with a spoon and discard the skins. Pass the potato through a fine drum sieve to produce a smooth purée.
Wash the parsley in several changes of cold water and drain. Blanche the parsley in boiling water for 15 seconds and refresh in iced water. Drain the parsley and dry on a clean kitchen cloth. Put the parsley into a food processor with 100 ml. water or chicken stock and blend on a medium speed. Continue to add more water or chicken stock as necessary and blend until a smooth, thick purée is achieved. Place a piece of muslin cloth over a bowl and secure with a piece of string. Pour the parsley purée into the muslin and allow to drain. When the liquid has drained off remove the purée from the muslin and place in a bowl. Gradually add the potato purée; the purée should remain firm and have an intense, deep, green colour. Add the garlic purée and truffle oil to taste and season with salt and freshly ground pepper. Cover with clingfilm and refrigerate until ready to use. Use the parsley purée within 48 hours, as it will begin to lose flavour and colour.