Combine peanuts, sugar, syrup and salt in a 1 1/2 quart microwave-safe bowl. Cook 8 minutes on high, stirring well after 4 minutes (cooking time may vary by oven). Stir in butter and vanilla.
Microwave 2 minutes longer on high. Add baking soda and quickly stir until light and foamy. Immediately pour onto lightly greased baking sheet.
Do not spread with a spoon to maintain the light, airy texture. Cool completely and break into pieces. Store in airtight container.