Have all ingredients measured out before starting.

Grease a large rimmed baking sheet. Set aside.

Combine sugar, corn syrup, water, and salt in a heavy, 2-quart saucepan over medium heat.

Stir until sugar is dissolved and the mixture comes to a boil, about 5 minutes.

Stir in peanuts and set a candy thermometer in place. Continue cooking, stirring occasionally, until the temperature reaches 300 to 310 degrees F (150 to 155 degrees C)

Remove from the heat. Quickly stir in butter and baking soda, then immediately pour mixture onto the prepared baking sheet. Quickly use 2 forks to lift and pull the mixture into a 12×14-inch rectangle. Let cool until completely firm, at least 30 minutes.

Use a mallet to break peanut brittle into pieces.