1. Line a large rimmed baking sheet with parchment paper and spray parchment paper with nonstick cooking spray.
2. In a dry frying pan, toast walnuts over medium heat for 3-4 minutes until fragrant. Remove from pan to a plate so they do not continue cooking.
3. In a medium saucepan over medium-high heat, combine sugar, corn syrup, butter, cinnamon, cloves, and water. Stir until sugar is dissolved.
4. Set candy thermometer in place, and continue cooking at a rapid boil without stirring or disturbing until mixture is an amber color, about 20 minutes or until candy thermometer reads 300°F.
5. Remove from heat and whisk in baking soda (it will bubble).
6. Stir in chopped walnuts, working quickly, and then immediately scrape brittle onto prepared baking sheet, spreading it out into an even layer.
7. While the brittle is cooling, sprinkle with flaked sea salt, if desired.
8. Let cool completely before breaking into pieces and storing in an airtight container.
Tip: Spray measuring cup with nonstick spray before measuring corn syrup – it will slide right out of the cup!