Place the eggs, extra yolk, vanilla and sugar in a heatproof bowl. Place over a saucepan of simmering water and, using a hand-held electric mixer, beat for 6–8 minutes or until thick and pale. Remove from the heat and allow to cool slightly. Whisk the cream until stiff peaks form. Gently fold through the egg mixture until well combined. Spoon lemon curd into the bases of 8 x ½ cup-capacity (125ml) glasses. Top with the semifreddo and freeze for 6 hours or overnight. Makes 1 litre.