Prepare Baking Sheet: Line a baking sheet with parchment paper. This will prevent the candied walnuts from sticking and make cleanup easier. Set the prepared baking sheet aside.
Combine Ingredients in Skillet: To a large non-stick skillet, add the walnuts, granulated sugar, brown sugar, water, and unsalted butter. If you are using cinnamon, add it to the skillet as well.
Cook Over High Heat: Place the skillet over high heat and bring the mixture to a boil.
Evaporate Moisture and Candy Nuts: Cook, stirring occasionally, until all the liquid has evaporated from the skillet and the walnuts become glossy and start to clump together. This process should take approximately 5 minutes. Keep a close eye on the nuts to prevent them from burning. The sugar will go through stages, from liquid to syrupy to a drier, candy-like coating.
Stir in Salt: Once all the moisture has evaporated and the walnuts are nicely coated and clumping, stir in the salt.
Transfer to Baking Sheet: Quickly transfer the hot candied walnuts from the skillet to the prepared parchment-lined baking sheet.
Spread and Cool: Spread the candied walnuts out into a single layer on the parchment paper, trying to separate them as much as possible to prevent them from sticking together in large clumps.
Harden and Break Apart: Let the candied walnuts sit at room temperature to cool completely. As they cool, the sugary coating will harden and become crisp. Once fully hardened, break the candied walnuts apart into smaller pieces.
Enjoy: Enjoy these quick and delicious 5-Minute Candied Walnuts as a snack, a topping for salads or desserts, or a delightful addition to a cheese board! Store them in an airtight container at room temperature.