Grease a large baking sheet and set aside
In a heavy 2-qt saucepan, combine sugar, corn syrup, salt and water and bring to a boil. Stir until sugar is dissolved.
Stir in peanuts.
Place candy thermometer in pot and make sure you can read it in the spot it's in.
Stir frequently until temperature reaches 300°.
Remove from heat and immediately stir in butter and baking soda and pour into prepared baking sheet. Using a spatula, try shaping the mixture into a rectangle.
Allow to cool completely at room temperature
Break into pieces and store in an airtight container.