Combine peanuts, sugar, corn syrup, and salt in a 1-1/2-quart casserole dish. Microwave on High for 7 to 8 minutes, stirring well after 4 minutes. Add butter and vanilla to syrup, blending well. Microwave for 1 to 2 minutes more. Peanuts should be lightly browned and syrup very hot. Add soda and gently stir until light and foamy. Pour mixture onto lightly greased cookie
sheet; let cool for 1/2 to 1 hour. When cool, break into small pieces and store in airtight container.