Line a cookie sheet with parchment or a silicone mat and spray with nonstick cooking spray.
Mix the sugar, corn syrup, and salt together in a 3-quart microwave-safe glass bowl.
Put bowl into microwave, uncovered, and cook for 4 minutes on full power.
Remove from the microwave very carefully and stir in the peanuts, ensuring they are all fully coated.
Return to the microwave and cook for an additional 3 minutes.
Add the butter and stir well to combine, then return to the microwave for 1 more minute.
Remove from the microwave and carry to where your prepared sheet pan is.
Stir in the vanilla and baking soda gently. The mixture will foam up.
As soon as all the ingredients are fully combined, pour it carefully onto the prepared baking sheet, scraping it out of the bowl with a heat-safe silicone spatula, and let it settle into a level sheet.
Let cool at room temperature for an hour, then break into pieces and store in an airtight container.
