Combine butter, baking soda, and vanilla in a bowl. Set aside.
Combine sugar, Karo Syrup, and water in a large saucepan over medium heat. Stir constantly and bring to a boil until mixture spins a thread when drizzled from your spoon. Sounds crazy, but you'll know what I am talking about when it happens. See the step-by-step video for an example.
Add peanuts and continue stirring the boiling mixture until it turns brown, like in the pictures, (approx. same color as a paper bag).
Add the bowl of butter, baking soda, and vanilla. Mix until butter is melted and mixture is foamy from the baking soda.
Pour out onto prepared cookie sheet and spread mixture out with two forks as thinly as you can (approx. ¼").
Cool completely.
Use a butter knife to break it into pieces. (See picture below.)
Recipe by Real Life Dinner at https://reallifedinner.com/moms-best-peanut-brittle-recipe/