Line a rimmed baking sheet with parchment paper. In a medium, heavy saucepan (not nonstick) over medium-high heat, combine the sugar, corn syrup, and peanuts and cook, stirring constantly with a heatproof spoon, until the peanuts begin to turn light brown and start to give off a nutty aroma, 7 to 10 minutes.
Remove the pan from the heat, add the baking soda and salt, and stir well. Be careful: The hot mixture will briefly foam up a bit.
Working quickly and wearing oven mitts or using potholders, pour the mixture onto the prepared pan and spread out with a spoon. Sprinkle with more salt if you like. Let cool, then break into bite-size pieces.