Add the unsalted butter, brown sugar, and salt to a small pan. Heat over medium heat until the butter has melted and the mixture begins to bubble around the edges. Mix in the walnuts and cook for 2-3 minutes until they are coated in the brown sugar mixture. Spread the walnuts out onto a baking sheet lined with parchment paper. Allow the candied walnuts to cool for at least 10 minutes.