Double line a 900g/2lb loaf tin with cling film.
For the ganache, melt the chocolate and butter in a large heatproof bowl over a saucepan of simmering water (do not allow the bottom of the bowl to touch the water). Stir in the icing sugar and double cream. Set aside.
Meanwhile, place the egg, egg yolks, vanilla extract and sugar in a heatproof bowl and set it over a saucepan of simmering water, making sure the bowl doesn’t touch the water. Beat the mixture over the heat for about 8 minutes using a hand-held electric whisk, until it is thick and pale. Remove from the heat and set aside.
In a separate large bowl, whip the double cream until it is just stiff. Gently fold in the pale egg mixture until combined.
Pour half the mix into the prepared loaf tin. Make a shallow well along the centre and pour in the ganache. Pour the rest of the semifreddo mix over the top and use a knife to make swirls with the ganache. Cover with cling film and place in the freezer to set for 6 hours.
Just before serving, melt the chocolate pieces in a bowl standing over a pan of simmering water (do not allow the bottom of the bowl to touch the water). Set aside to cool to room temperature.
To serve, turn the semifreddo out onto a serving platter and carefully remove the cling film. Pour the cooled chocolate over the top, sprinkle with the toasted hazelnuts and serve in generous slices.