Combine sugar, corn syrup, and salt in saucepan and bring to a boil over medium-high heat. Stir constantly and boil until mixture looks clear and is free from sugar crystals. Remove pan from heat and stir in peanut butter and vanilla. Stir till smooth and free from lumps. Add cereal and cashews, if using, and mix to combine.

Use your hands to form into small balls and place on waxed paper or foil. Clusters will harden as they cool. After 15 to 20 minutes, transfer to an airtight container.