In a glass bowl, combine peanuts, sugar, corn syrup, and salt. Cook in microwave for 6 to 7 minutes on High power. Mixture should be bubbly and peanuts browned.
Stir in butter and vanilla then cook 2 to 3 minutes longer. ( see note)
Quickly stir in baking soda (just until mixture is foamy). Pour immediately onto greased baking sheet.
Allow cool 15 minutes or until set. Break into pieces (I just use a butter knife to crack it apart), and store in an airtight container.
If you know that your microwave tends to cook faster and hotter (I've had a microwave like this before, it always cooked fast and very hot!) then you should reduce cooking time by a bit so you don't burn it.