Place the nuts onto a lined tray and into a cold oven set to 190°C and roast for 14 minutes. You want a dark roast without burning. Cool and roughly chop.
Place butter, sugar, water and vanilla into heavy-based saucepan and bring to a simmer on a medium heat (induction 6). Stir occasionally and continue to cook until it reaches hard crack stage (150°C/305ºF on candy thermometer).
Pour over onto a silpat mat, working quickly to spread. Sprinkle with chopped nuts and salt. Allow to cool completely before breaking into pieces. Store in an airtight container in the fridge. This will make it all the more crunchy.