In a small saucepan, combine brown sugar, cinnamon, salt, and water. Stir until combined and melted.
Add hazelnuts and stir to coat. Cook for 2-3 minutes over medium heat until mixture reduces slightly and hazelnuts look sticky.
Pour nuts onto a sheet of parchment and spread into a single layer. Allow to cool. Store in a sealed container until ready to use.
If you have raw or blanched hazelnuts, or roasted hazelnuts that are not fragrant, you will want to roast them before candying.
To roast in the oven: Preheat to 350 degrees Fahrenheit. Spread nuts onto a baking sheet in a single layer and bake for 10-15 minutes, or until fragrant and lightly browned. You'll want to check your nuts halfway through to make sure they're roasting evenly.
To roast on the stovetop: In a skillet over medium heat, add your nuts (do not add any additional oil - your nuts will not stick to the pan). Cook until nuts are hot and fragrant, stirring occasionally. This should take up to 5 minutes.