In a large skillet mix the sugar, syrup, salt and butter together. Cook at a medium-high heat, stirring regularly. Heat until the sugar has dissolved. Stir in peanuts. Heat to 290 degrees F on a candy thermometer.
Remove from heat and stir in baking soda.
Pour mixture onto greased cookie sheet as one big piece and broken into smaller pieces once cooled. Note: Do not tip the cookie sheets to thin the brittle - it is best thick.