Combine eggs, extra yolks, vanilla extract and Chelsea Caster Sugar in a heatproof bowl and place over a saucepan of simmering water, whisk for 5 minutes or until frothy. The heat helps to increase the volume of the whipped mixture.
Remove from heat and beat with an electric mixer until thick and pale. Lightly crush 300g of the mixed berries and the pistachios and fold through egg mixture.
Beat cream with an electric mixer until soft peaks form. Gently fold through egg mixture.
Line a 26cm x 10cm loaf pan with plastic wrap. Lay remaining berries at base of the pan.
Pour over the semifreddo mixture. Cover with foil and freeze overnight.
Remove from pan and decorate with berries. Cut into slices and enjoy!