Generously butter a cookie sheet. In a 2-quart (8 cup) glass mixing bowl, using a spoon, stir together granulated sugar and corn syrup. Microwave mixture on HIGH power 4 minutes.
Stir in peanuts and salt, microwave on HIGH power 3 1/2 minutes.
Mix in butter and vanilla extract, stirring until butter is melted. Microwave on HIGH power 1 1/2 minutes*.
Stir in baking soda (mixture will become fluffy and foamy). Pour and and lightly spread out onto prepared baking sheet.**
Allow to cool at room temperature then break into pieces once set.
Cook times may vary slightly depending on microwave wattage. Watch for a golden brown color near the end. Newer higher wattage microwave ovens may need about 30 seconds or so cut from the time.
Note that the less you spread the lighter the brittle will be, or more air bubbles it will have, so if you prefer it like so then do as little spreading as possible. If you prefer it flatter then spread it out thin.