In a glass bowl, combine peanuts, sugar, 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 bicarb, just until mixture is foamy. Pour immediately onto greased baking tray. Let cool 15 minutes, or until set. Break into pieces, and store in an airtight container.