Combine peanuts, corn syrup, sugar, and salt in a 1-1/2- to 2-quart glass measuring dish, preferably one with a handle, like a big measuring cup. Microwave on High for 8 minutes, stirring with a wooden spoon every 2 minutes.

Add butter and vanilla. Microwave on High 2 minutes longer, without stirring. Add baking soda and stir.

Working quickly, pour candy mixture onto buttered cookie sheet, in an even layer, distributing peanuts as evenly as possible. Cool completely.

Break into 2- to 3-inch pieces and store in an airtight container.