Combine sugar, Karo syrup, water and salt into a heavy, deep sauce pan. Stir until dissolved and using a candy thermometer bring it to a temperature of 238 degrees F.
Add in butter and peanuts. Stir often, it burns easily. Cook to 287 degrees F.
Remove from heat and add baking soda and vanilla. Stir as little as possible.
Pour onto a large buttered baking sheet with edges. Just allow it flow onto the pan, do not try spreading it.
Allow to cool and then break into pieces. Stir in an airtight container.