Line a rimmed baking sheet with parchment paper and set aside.

Measure and prepare the peanuts, butter, baking soda, and vanilla in advance, keeping them within reach of the stovetop.

In a medium saucepan, combine the sugar and water, stirring until evenly mixed. Add the corn syrup and cook over medium heat, stirring occasionally, until the mixture comes to a gentle boil.

Attach a candy thermometer to the side of the saucepan, ensuring it is submerged in the syrup but not touching the bottom. Continue cooking, stirring occasionally, until the temperature reaches 250°F (121°C).

Stir in the peanuts and cook, stirring constantly, until the thermometer reaches 300°F (149°C).

Immediately remove the saucepan from the heat and stir in the butter, baking soda, and vanilla. The mixture will foam and lighten in texture.

Carefully pour the hot candy mixture onto the prepared baking sheet. Using a heat-resistant spoon or spatula, spread the brittle into an even layer as quickly as possible.

Allow the peanut brittle to cool completely for at least 30 minutes, then break into pieces.

Store the cooled peanut brittle in an airtight container at room temperature.