Heat the butter in a heavy bottom skillet over medium heat.
Add the walnuts and sugar, tossing to coat. Allow the sugar to melt and become smooth. Continue to heat until it isn't gritty any longer. If it starts to burn, reduce heat to low.
Spray a piece of parchment paper with cooking spray.
When the walnuts are evenly coated, turn them out onto parchment in a single layer. Sprinkle them with the cinnamon or fleur de sel (both optional) while they are still tacky.
Allow the walnuts to cool and sugar to set, then crumble using your hands.
If you've tried this recipe, come back and let us know how it was in the comments!