Grease a square tin and line with baking paper.
Whisk the miso, honey, double cream, caster sugar, light brown sugar in a pan until completely dissolved. Add in the butter and bring to a boil.
Cook for 15 mins or until a thermometer inserted reads 125°C. To check its ready, add a small amount into a glass of cold water. If it's the right temp, you should be able to roll it up into a soft ball.
Pour into the lined square tin, and tap it on the surface a few times to remove the bubbles. Sprinkle liberally with sea salt and allow to cool fully (will take around 2 hours).
Slice into bite-sized pieces and enjoy.