Lightly grease the inside of a loaf tin, then line it with 2 layers of cling film leaving overhangs on each sides.
Break chocolate into pieces and melt in a heat proof bowl over a pan of simmering water. Stir well to make sure all chocolate has melted. Leave aside to cool for 5 minutes.
In a large bowl, whisk eggs and sugar for 3-4 minutes until pale, thick and fluffy.
In another bowl, whisk thickened cream and vanilla extract to a soft peak.
Stir cooled, melted chocolate into the egg mixture.
Add whipped cream into the chocolate mixture and mix well. Fold in almonds.
Pour mixture into loaf tin, and cover with overhanging cling films. Freeze for at least 4 hours. It can stay in the freezer for even about 2 weeks before serving.
Remove from the freezer 10 minutes before serving to soften a little. Run a table knife around the inside edge of the tin, then peel back the top cling films and turn the semifreddo onto a serving plate. Remove the rest of the cling film.
Sprinkle some grated chocolate if desired, slice the semifreddo and serve.