Grease a baking sheet, and set aside. In a glass bowl, combine peanuts, sugar, corn syrup, and salt. Cook in microwave for 6 to 7 minutes on High (700 W); mixture should be bubbly and peanuts browned. Stir in butter and vanilla; cook 2 to 3 minutes longer.
Quickly stir in baking soda, just until mixture is foamy. Pour immediately onto greased baking sheet. Let cool 15 minutes, or until set. Break into pieces, and store in an airtight container.