Set out a large ceramic microwave-safe bowl. (Not metal or plastic.) Then lay a large piece of parchment paper out on the countertop.
In the bowl, mix the sugar and corn syrup into a thick paste. Microwave on HIGH for 4 minutes.
Add the peanuts to the syrup, and carefully stir them in. Microwave again for 3 minutes. (Boiling sugar syrup can be dangerous, so watch your fingers!)
Stir in the butter and vanilla extract. Microwave for 1 ½ minutes. At this point, the syrup should be golden in color like honey, not too pale or too golden-brown. If it looks pale, microwave another 1-2 minutes.
Finally, quickly but carefully stir in the baking soda. The mixture should foam up and create bubbles. These bubbles are VERY important for the light airy texture of the brittle. Try not to deflate them.
Quickly pour the brittle mixture onto the parchment paper. Pour ribbons in a circular or zigzag pattern, so the brittle spreads on its own. If you try to spread it with a spatula is will deflate the ever-important bubbles. Shake the paper a little to help the spreading, but do not press on the brittle.
Once the brittle is fully cooled, break the brittle into pieces. Store in an airtight container for up to 4 weeks.
Nut brittles will keep well for up to 3 weeks stored in an airtight container at room temperature. You can also store in a sealed container and keep in the freezer for up to 3 months.