Preheat oven to 350 degrees. Spread walnuts into an even layer on rimmed baking sheet. Toast 8 to 10 minutes, rotating sheet halfway through, until nuts are golden. Keep warm. Line another rimmed baking sheet with a nonstick baking mat or oiled parchment paper.
Place butter and sugar in medium saucepan over high heat. Cook, stirring occasionally, until butter is melted.
Continue cooking without stirring, swirling pan occasionally, until mixture turns deep amber, 8 to 10 minutes.
Quickly stir in roasted nuts and salt and pour out onto prepared baking sheet. Allow to cool completely, then coarsely chop.