Line a loaf tin with plastic wrap, making sure there is sufficient over lap on the sides. You can also and a few strips of baking paper across the base and side of the tin to help lift out the semi freddo, but don’t use baking paper to line the tin, it must be plastic wrap.
Mash half of the plums with a spoon until mushy and the big pieces are broken up. Reserve the other half for garnish.
Whip the cream until stiff and fold in the 1/4 cup of castor sugar and set aside, preferably in the fridge
In the bowl of a stand mixer place the eggs, and whisk until light in colour and more than doubled in volume, slowly add in the 1/2 cup of vanilla sugar until thick ribbon stage. Fold in the cardamon powder, and very gently fold in the whipped cream carefully keeping as much air in the mixture as possible.
Transfer to the lined tin and top with the mash plums, using a wooden skewer and swirl the plums through the cream mixture until marbled.
Place in the freezer for 24 hours.
To serve unmold and serve top with the remaining roast plums and garnish with toasted nuts ,sesame seeds and dried rose petals