Line a large, rimmed baking sheet with a nonstick baking mat, parchment or wax paper. If using parchment or wax paper, lightly spray with nonstick cooking spray. Set aside.

Combine the butter, sugar, corn syrup, water and salt in the medium saucepan and attach a candy thermometer. Place the pan over medium heat and cook, stirring frequently with a rubber spatula, until the mixture is dark amber and reaches 300ºF (the hard crack stage), 20-25 minutes.

As soon as the mixture reaches 300ºF, remove the pan from the heat and remove the thermometer. Stir in the hazelnuts and pour the mixture onto the prepared baking sheet. Use the spatula to spread it out to an even thickness (about ¼-inch).

Allow the mixture to cool at room temperature for about an hour. Break up the toffee into large chunks and store in an airtight container at room temperature. It will keep a couple weeks, but you won’t need it to last that long. Trust me.