Stir 200gm sugar and 100ml water in a saucepan over medium-high heat until sugar dissolves, bring to the boil and cook until dark caramel (5-6 minutes). Add butter (be careful, hot caramel will spit) and swirl to combine. Add pouring cream, stir, then sea salt to taste (you’ll need more than you think), and refrigerate to cool completely.
Whisk yolks and remaining sugar in a bowl over a saucepan of simmering water until thick and pale (6-7 minutes). Transfer to an electric mixer and whisk until cooled to room temperature. Fold in salted caramel, then thickened cream, transfer to a container and freeze overnight.
For black sesame sugar, process ingredients and 1 tbsp sea salt in a food processor to a sand-like texture. Store in an airtight container in the pantry for up to 2 weeks.
Serve scoops of semifreddo in chilled bowls, scattered with black sesame sugar.