In a microwave safe bowl, combine peanuts, sugar, corn syrup and salt. Cook in microwave 4 minutes; remove and stir; return to microwave for 4 more minutes. Stir into peanut mixture, butter and vanilla. Microwave 2 minutes.
Immediately after removing mixture from the microwave add baking soda. Stir until foamy. Spread mixture on a greased cookie sheet. Cool until hard. Break into pieces.