Add the butter, brown sugar, and a pinch of salt to a small non-stick skillet. Heat over medium until the butter is melted and the brown sugar is just beginning to sizzle around the edges (it should still look grainy).
Add the walnuts (chopped or whole) and stir to coat in the brown sugar mixture. Continue to cook and stir over medium heat for 2-4 minutes, or until the brown sugar is fully melted and coating the walnuts (it should look glassy, not grainy).
Remove the skillet from the heat immediately, as the sugar can go from melted to burning quickly. Spread the candied nuts out over a sheet of parchment paper, making sure they are in a flat layer and separated as much as possible.
Allow the nuts to cool fully (about 10 minutes). The candy coating will fully harden as they cool.