Preheat the oven to 180°C, gas mark 4. Grease 2 large baking trays and line with baking paper.
Put the parsnip in a saucepan, cover with water and bring to the boil. Reduce the heat and simmer for 8-10 minutes until very soft. Drain and mash until smooth and lump free.
Whisk the butter and sugar until light and fluffy, then beat in the mashed parsnip and egg. Stir in the orange zest, 100g white chocolate, cranberries, hazelnuts and nutmeg.
Sift over the flour and baking powder and gradually stir in, mixing well.
Drop heaped teaspoonfuls of the mixture onto the prepared trays and press lightly to flatten slightly, then bake for 15-20 minutes, until golden. Transfer to a wire rack and leave to cool completely.
Put the remaining chocolate in a heatproof bowl set over a pan of barely simmering water and leave to melt, stirring every so often. Drizzle the chocolate over the cookies and leave to set before eating.