Pre-heat one sous vide machine to 70C and a second sous vide machine to 30C.
For the Vanilla Semifreddo with Cherries Soaked in Cognac, combine 120g of the sugar with 50g of the water in a 1 litre copper saucepan over medium heat, stirring occasionally until sugar melts and the mixture reaches 118C.
Meanwhile, place egg yolks in the bowl of a stand mixer fitted with a whisk attachment and whisk until pale.
Once the sugar syrup has come to temperature, pour the mixture slowly into the egg yolks with motor running to make a pâte à bombe. Continue to whisk to cool the mixture, until needed.
Meanwhile, make an Italian meringue by combining the remaining water and sugar into a small copper saucepan over medium heat, stirring occasionally until sugar has dissolved and the mixture reaches 118C.
Meanwhile, place the egg whites in the bowl of a stand mixer fitted with a whisk attachment. Whisk egg whites on medium speed until soft peaks form. Once the sugar syrup has come to temperature, pour slowly into the egg white with motor running and until mixture is thick and glossy, to make an Italian meringue. Continue to whisk until the mixture has cooled.
Once cooled, add Italian meringue into the pâte à bombe mixture and continue to whisk on medium speed until combined, taking care not to over work the mixture. Set aside until needed.
Place the thickened cream into the bowl of a stand mixer fitted with a whisk attachment. Add the scraped out vanilla seeds and whip to soft peaks.
Working quickly, gently fold the whipped cream through the egg yolk mixture until just combined, taking care not to knock out too much air. Add the chopped cherries and gently fold in until mixed through.
Place a sheet of 7cm half sphere moulds on a flat tray. Spoon the mixture into the moulds and use an offset spatula to level off the surface. Place the moulds in the blast chiller until completely frozen.
Once the semifreddo is frozen, remove from the blast chiller and carefully unmould. Using the palm of your hand, gently warm the flat side of two half spheres and join together to form a ball. Use fingertips to gently smooth the seam.