Attach the candy thermometer to the pot and combine the sugar, Karo syrup, and water. Heat the mixture in the saucepan until it reaches 236°F (113°C) on the thermometer, stirring occasionally with a wooden spoon.
When it reaches 236°F (113°C), add the peanuts and continue to cook until the temperature reaches 294°F (145°C) on the thermometer, stirring frequently.
Grease the sheet pan with butter. When the candy reaches 294°F (145°C), remove it from the heat and stir in the salt and baking soda.
Keep stirring; the candy will start to puff up. Once all the ingredients are incorporated, quickly pour the mixture into the greased sheet pan and spread it out evenly.
Allow it to cool completely, then break it into pieces. Store in an airtight container.
Notes
If you want to make a lot you will have to restart the recipe over. Doubling up the recipe and the candy will not come out right.