Line a baking sheet with parchment paper and set it aside. Have a spatula ready to spread the brittle once it’s cooked.
In a large, microwave-safe glass bowl, combine the sugar, corn syrup, and salt. Stir until the mixture is evenly combined.
1 cup granulated sugar, ½ cup light corn syrup, ¼ teaspoon salt
Microwave the mixture on high for 4 minutes. The mixture will start to bubble and turn light golden.
Carefully remove the bowl from the microwave (it will be very hot). Stir in the butter, peanuts, and vanilla extract until fully melted and incorporated.
1 cup peanuts, 1 tablespoon unsalted butter, 1 & ½ teaspoon vanilla extract
Return the bowl to the microwave and cook for another 2-3 minutes until the mixture turns a deep golden brown. Keep a close eye on it to avoid burning.
Quickly stir in the baking soda. The mixture will foam and bubble up, which is normal. Stir until the baking soda is fully mixed in.
1 teaspoon baking soda
Immediately pour the mixture onto the prepared baking sheet. Use a spatula to spread it out into a thin layer as quickly as possible.
Let the brittle cool completely, about 30 minutes. Once it’s hardened, break it into pieces.
Store the peanut brittle in an airtight container at room temperature. It will keep for up to 3 weeks.