Start by combining the granulated sugar and light corn syrup in a large microwave-safe bowl. Stir well until all the sugar is wet, ensuring there are no dry lumps. This mixture will form the base of your brittle.
Place the bowl in the microwave and heat on high for four minutes. Keep an eye on it, as the mixture will bubble vigorously. The color should turn a light golden hue, indicating that the sugar is caramelizing.
After four minutes, carefully remove the bowl from the microwave. It will be hot, so use oven mitts! Add in the vanilla extract, peanuts, and butter. Stir the mixture well to ensure that the peanuts are fully coated.
Return the bowl to the microwave for an additional four minutes. This second round will give the mixture a deeper caramel color and richer flavor. Again, watch closely to avoid any overflow.
Once done, quickly stir in the baking soda. This will cause the mixture to bubble and expand, creating air pockets that make the brittle light and crispy. Be cautious, as the mixture is still hot.
Prepare a greased cookie sheet by spraying it lightly with cooking spray or lining it with parchment paper. Pour the hot mixture onto the sheet and spread it out evenly with a spatula or wooden spoon. Work quickly, as the mixture will start to cool and harden.
Allow the brittle to cool completely at room temperature. This might take about half an hour. Once it has cooled and hardened, you can break it into pieces of your desired size.
Enjoy your homemade Microwave Peanut Brittle! It can be stored in an airtight container for weeks, though I bet it won't last that long!