Prepare Pan: Line a 9 inch x 5 inch (22cm x 12cm) rectangle baking tin with two layers of cling film, leaving extra to hang over the sides.
Double Boiler Setup: Use a small-medium size heat-proof bowl over a saucepan of simmering water as a double boiler.
Melt Chocolate: Add broken up chocolate to the bowl, place it over the saucepan and stir occasionally until melted and smooth. Remove and set aside to cool.
Combine Egg Ingredients: In a large heat-proof mixing bowl, add the egg yolks, whole egg, sugar and vanilla.
Heat Over Water: Using the same double-boiler method & saucepan as the chocolate, place the bowl over the saucepan, ensuring the base of the bowl doesn’t touch the water. The water must be simmering.
Beat Mixture: Using an electric beater on low speed, beat the egg mixture for 4-5 minutes, while it’s in the double boiler, until the mixture has doubled in volume and is thick, pale, and leaves a trail after you lift the beaters up. Remove the bowl from the saucepan, turn off the heat and set aside.
Whip Cream: In a separate mixing bowl, beat the heavy cream to firm peaks, about 4 minutes on medium-high speed. Use clean whisk attachments or wash the ones you used for the egg mixture.
Fold in Chocolate: Gently fold the cooled melted chocolate into the whipped egg mixture, do this gently to avoid losing too much volume. Note: You may find some tiny bits of where the chocolate has turned solid, this is ok, more texture!
Add Cream: Gently fold the whipped cream into the chocolate egg mixture until well combined and no pockets of cream remain.
Transfer to Tin & Cover: Pour the mixture into the plastic lined tin, smoothing the top. Fold over the overhanging cling film or add new cling film to cover the semifreddo.
Set in Freezer: Freeze for 6-8 hours or ideally overnight. The semifreddo will be set but not completely hard.
Unmold: Remove the semifreddo by inverting the pan onto a platter and peeling away the cling film.
Garnish: Top with cherries and fresh mint. Slice, serve and enjoy! Tip: If too solid, let it sit at room temperature for 5-10 minutes before slicing.