Generously oil a baking sheet and set aside.

Bring sugar and rum to boil in a saucepan. Cook for 3 minutes, until candy thermometer reads 234. 

Remove from heat and add cashews, stirring constantly to coat with syrup. The sugar will become white and grainy after about 30 seconds.

Return saucepan to heat and re-melt sugar, adding a few drops of water if needed. Watch carefully - do not burn.

When syrup has turned a rich caramel color, about 2 minutes, and the nuts sound hollow at the tap of a spoon, remove from heat and stir in butter.

Transfer mixture to prepared baking sheet, spreading out in one layer, separating any pieces that are stuck together. Cool completely.

Store airtight for 2 weeks.

You will need a candy thermometer for this one.