Coat a baking sheet with butter. Set aside. Mix sugar and Karo Syrup together in a microwave safe glass bowl. Microwave for 3 minutes. Mixture will be hot and bubbly. Carefully remove.
Add in butter, vanilla, and cashews and stir to combine. Microwave another 2 minutes. Carefully remove.
Stir in baking soda. Mixture will become fluffly. Quickly pour onto prepared baking sheet. Sit at room temperature for one hour before serving.
Please use caution and pot holders when removing glass bowl from microwave. It will be very hot.
Also use caution when pouring onto baking sheet. Work quickly as mixture will want to set up almost immediately.