Preheat the oven to 170°F. Line a large cookie sheet with parchment paper and place it in the oven to keep warm. A warm pan makes spreading the brittle easier.

In a large saucepan over medium heat, combine the sugar, corn syrup, salt, and water. Stir continuously for 3 minutes, or until the sugar is fully dissolved.

Add the peanuts and stir continuously until the temperature reaches 300°F on a candy thermometer. This is the “hard crack” stage, where the brittle sets properly once cooled.

Remove from heat and quickly stir in the butter and baking soda, then immediately pour the mixture onto the prepared cookie sheet. Use a buttered spatula to spread it into an even layer.

Let the brittle cool completely. Once set, lift it gently from the pan and snap the candy into pieces by hand. Store in an airtight container at room temperature.