Prepare your baking sheet by lining it with parchment paper and set aside.
In a medium saucepan, combine granulated sugar, light corn syrup, and salt. Stir until well combined.
Place the saucepan over medium heat and cook the mixture, stirring occasionally until it reaches 240°F (soft ball stage) on the candy thermometer.
Remove the saucepan from heat and carefully stir in the butter, peanuts, and vanilla extract. Mix until well combined.
Add baking soda to the mixture. Be cautious as it may bubble up. Stir until the mixture is smooth and evenly mixed.
Immediately pour the mixture onto the prepared baking sheet. Use a spatula or wooden spoon to spread it evenly, coaxing it into your desired thickness.
Allow the brittle to cool completely at room temperature. This should take about 30 minutes to 1 hour.
Once cooled, break the brittle into pieces and enjoy!