Add the sugar, water, and karo corn syrup to a large soup pot and cook on stove at medium high heat (about a 7). Cook the mixture, stirring constantly, till it spins a thread (If you're not sure what spinning a thread is, it's when you dip the stirring paddle in the mixture, then pull it out and let the mixture drip off the spoon. When you see it start to ribbon off like a spider web rather than just drip straight down, it is spinning a thread).
Once the mixture spins a thread, add the peanuts. Cook the peanuts (stirring constantly) till the mixture turns a light golden color and the peanuts look roasted (be careful not to burn). This should take approximately 8 - 10 minutes.
Once the peanuts have turned golden brown, add the butter, baking soda, salt, and vanilla extract and stir quickly till the butter is completely melted.
Pour the mixture onto 2 well greased baking sheets and allow to cool for 45 minutes. Once the mixture is cooled, break apart the brittle and enjoy!