Heat the oven to 180C/fan 160C/gas 4. Scatter the nuts onto a baking sheet and toast in the oven for 5-10 minutes, tossing regularly until brown, then spread evenly on a piece of baking paper on a large baking tray.
Put the sugar, butter and 125ml water in a large pan and heat gently until melted. Turn up the heat until the mixture is bubbling, then leave to cook until it has turned a rich golden brown – this will take about 20 minutes, depending on the heat.
Keep a close eye on the pan and give it a stir every now and then. Once the mixture is golden brown, remove from the heat and pour straight over the cashews.
Sprinkle generously with sea salt flakes. Leave to set before breaking into shards and packaging it.