Tip125g (4½oz) caster (superfine) sugar into a frying pan and heat over medium–low heat. Cook until the sugar dissolves and turns a deep caramel colour. Swirl the pan, but do not stir. As soon as the sugar turns into caramel, take the pan off the heat.
Add80ml thick (double) cream and 25g (1oz) unsalted butter and mix until well combined.
Add2tablespoonswhite miso to the pan and stir until the miso dissolves.
Transfer the sauce to a screw-top jar and store in the fridge. It’s great with ice cream.