Line a baking sheet with parchment paper. Lightly spray with nonstick cooking spray. Set aside.
In a large microwave glass bowl add the sugar, peanuts and Karo syrup. Microwave on high for 4 minutes.
Remove from microwave and stir well with a wooden or silicone spoon.
Return to microwave and cook for another 3 to 3 ½ minutes until the peanuts begin to turn light brown.
Remove from the microwave and stir in the vanilla and butter. It will hiss and sputter when you add the vanilla, that's ok! It's supposed to do that. Return to the microwave and cook on high for another minute.
Remove from the wave and stir in the baking soda. Stir until light and foaming. Pour onto the prepared cooking sheet and spread out.
Allow to cool completely. Break into pieces. Store in an airtight container at room temperature. Enjoy!
Video
Notes
When you spread out the peanut brittle it may not look "right" but don't worry, it will fill in and become smooth.Be sure to use hot pan holders when stirring the peanut brittle and working with the bowl, the molten sugar is incredibly hot! This should go without saying... but... DO NOT stick your finger into the peanut brittle until it has cooled.