Dice the stollen very finely and put 1 tablespoon aside for sprinkling. Chop the chocolate and pistachios. Set aside 1 tsp. each of pistachios and almonds for sprinkling.
Separate eggs. Beat egg whites and cream separately until stiff and cover and chill. Whisk egg yolk and sugar with the whisks of the mixer for 3-5 minutes until white and creamy. First fold in the stollen pieces, pistachios and almonds, then the cream and then the beaten egg white.
Pour the semifreddo mixture into eight timbal moulds or cups (each containing approx. 150 ml). Freeze for at least 3 hours, preferably overnight.
Remove Semifreddo about 15 minutes before serving (if necessary, refreeze the rest immediately). Chop white chocolate and melt in a hot water bath. If necessary, dip the moulds briefly in hot water and turn the Semifreddo out onto plates.
Pour over with liquid chocolate. Serve sprinkled with the rest of pistachios, almonds and pieces of stollen.