First make the truffle butter. Beat the butter and truffle paste together in a bowl until smooth. Form into a log, wrap in clingfilm and chill in the freezer for 30 minutes. Cut into slices.
Meanwhile, soak the porcini mushrooms in the measurement water for 15 minutes. Drain well, reserving the soaking liquid, then chop the porcini.
Melt half the butter in a saucepan, add the onions, garlic and thyme and cook over a low heat, stirring occasionally, for 10 minutes. Add the remaining butter and the fresh mushrooms and porcini and cook over a medium heat, stirring frequently, for 5 minutes, until the mushrooms are softened. Stir in the stock and the reserved soaking liquid and bring to the boil, then reduce the heat, cover and simmer gently for 20 minutes.
Transfer to a food processor or blender and process until really smooth. Return to the pan, stir in the cream and heat through without boiling. Spoon the soup into serving bowls and serve each portion topped with slices of the truffle butter and chopped chives.