Prepare a large 17” rimmed cookie sheet by spraying it with a nonstick cooking spray, spray the edges as well. Set aside.
In a deep skillet, slowly cook sugar, corn syrup, and 1 cup of water on medium-high heat, stirring until the sugar dissolves. bring to a boil and continue cooking until the candy thermometer reaches the soft-ball stage (235°F), about 10 minutes.
Add in the peanuts and salt. Cook until the candy thermometer reaches the hard crack stage (300 to 310°F), stirring constantly, about 10 more minutes. (Pay attention to this part as the brittle will burn if cooked too long).
Remove from heat and add in the butter and baking soda; stir to blend. The mixture will bubble up.
Pour the brittle mixture onto the prepared cookie sheet and spread it out with a spatula and allow the brittle to cool for about 30 minutes until it’s cool to the touch.
Once cooled completely, slide a spatula underneath to lift the brittle up and break it into pieces. Store covered at room temperature.