Combine the sugar and corn syrup in a large microwave-safe bowl; microwave on HIGH for 4 minutes.
Stir in the peanuts and butter; microwave for an additional 3 minutes or until the mixture turns a caramel color.
Add the vanilla and baking soda - working quickly to mix and spread the mixture onto the prepared baking sheet.
Let the candy stand until cool, and it hardens. Break into pieces.
Notes
Note that microwave ovens are different. You may need to adjust your time slightly. Keep an eye on your brittle and watch the color. You don't want it too pale and you don't want it too dark.
Once the brittle has cooled, break it with your hands by bending it slightly. If you're having trouble, you can use a kitchen mallet or other small heavy object to break it into pieces.
Be sure to grease your baking sheet very well with butter. This will help determine how easy getting the final candy off the pan is.
The sugar will be VERY HOT when making this recipe. Please take caution when handling it as not to burn yourself.