1. Generously butter a baking sheet. Combine nuts, corn syrup, sugar and salt in 1-1/2- to 2-quart microwave-safe dish. Microwave on high power 8 minutes; stir with wooden spoon every 2 minutes.
2. Add butter and vanilla. Microwave on high power 2 minutes, without stirring. Stir in baking soda. Working quickly, pour candy mixture in even layer onto the buttered sheet, distributing peanuts as evenly as possible. Transfer baking sheet to wire rack to cool completely.
3. Break brittle into 2- to 3-inch pieces. Store in an airtight container for up to 1 month. Makes 15 to 20 pieces.