Combine the eggs, yolks, vanilla extract and caster sugar in a heat−proof bowl. Place the bowl over a saucepan of barely simmering water and whisk the mixture with an electric whisk for 6−8 minutes or until it turns light and thick. Remove from the heat and allow to cool off slightly.
Carefully fold in the whipping cream into the egg mixture and mix until smooth. Pour in the melted chocolate, a little at a time, add the crushed praline and stir. Add the orange zest. Pour the mixture into a 2 litre tin or two smaller tins and freeze for 6 hours or overnight.
Allow the semifreddo to sit at room temperature before serving. Serve straight away, directly from the tin, or spoon into individual bowls.