Grease a 15x10x1-in. pan and a metal spatula; set aside.
In a 2-qt. microwave-safe bowl, combine sugar and corn syrup. Microwave, uncovered, on high for 4 minutes; stir. Cook 3 minutes longer. Stir in peanuts and butter. Microwave for 30-60 seconds or until mixture turns a light amber color (mixture will be very hot).
Quickly stir in cocoa, baking soda and vanilla until combined. Immediately pour into prepared pan; spread with the metal spatula. Cool before breaking into pieces. Store in an airtight container.