Bring a small saucepan or double boiler with water to the boil and then reduce to a simmer. Have an ice water bath big enough for the base of the double boiler or mixing bowl on standby.
In a medium glass bowl or the pan of a double boiler, combine the egg yolks, sugar, and saffron threads. Place the bowl or pan over the simmering water and whisk constantly until the mixture is thick and doubled in volume.
Remove from heat and submerge in the ice water bath, continuing to stir to bring down the temperature. If the mixture gets too cold and is sticking to the bottom of the pan, it will release easily if you run hot water on the outside of the bowl.
Beat the heavy cream in a mixer until soft peaks form. Set aside.
Place the egg yolk mixture and the sweet vermouth in the bowl of mixer with a whip attachment, and whip until the mixture becomes thick and pale in color.
Add a third of the whipped cream to the egg mixture and stir together gently. Fold in the remaining whipped cream into the egg mixture. When this is done, you can spoon the mixture into 6 individual ramekins, or place it in a plastic cellophane lined loaf pan, or decorative silicone mold of your choice. Cover the ramekins or other container with plastic wrap and freeze until firm, at least 6 hours. To serve, run a butter knife under hot water and cut around the edge of the ramekin to invert the semifreddo onto individual plates. Serve with the cherry-cardamom syrup and salted honey-hazelnuts (recipes to follow).
For cherry-cardamom syrup:
Combine all of the ingredients into a small saucepan and bring to a boil. Reduce the heat and simmer until the syrup is reduced by half and coats the back of a spoon. Set aside to cool.
For the salted honey-hazelnuts:
Preheat oven to 180C (350F) degrees.
Place the hazelnuts in a small mixing bowl and toss with the olive oil and sea salt. Add honey and toss once more to coat. Transfer the nuts to a silpat-lined baking sheet and bake, stirring at least once, until the nuts are golden, about 10 minutes. Remove from oven and let cool completely.
Once the hazelnuts have cooled, roughly chop the nuts and serve alongside the semifreddo.