Combine sugar, water and syrup in a medium-large saucepan. Stir to combine. Bring to a boil and cook at medium heat slowly to the soft ball stage (245 degrees F. on a candy thermometer), stirring gently every so often. Add 2 cups of peanuts and salt (if using). Continue to cook and stir until the thermometer registers 300 degrees F. Add baking soda. Stir to blend and continue to cook until golden brown (just a minute or so more). Remove from heat and stir to combine.
Pour the hot mixture onto the prepared pan and spread it out as much as you can with a wooden spoon. Cool completely, and then break into pieces.
*Keep your peanut brittle in a tightly covered container until you are ready to serve/eat it. The texture of candy can be greatly affected by the humidity in the air.
*If you are using a candy thermometer that clips to the side, just make sure that the bottom of the thermometer is resting inside the liquid and not pushed all the way to the bottom of the pan (you'll get a false reading if it's touching the bottom of the pan where the heat source is strongest).