In a large saucepan, combine the sugar, corn syrup and water. Cook over medium heat until the sugar dissolves, stirring constantly.
Once sugar dissolves, increase the heat to medium-high. Add peanuts and salt then cook until the temperature reaches 300 degrees or the hard crack stage, making sure to continuously stir.
Finally add in baking soda, butter and vanilla and mix well. Next, pour the brittle mixture onto 2 thoroughly greased baking sheets and spread to the thickness you desire.
Once completely cooled, crack into pieces and serve.
MAKE AHEAD + LEFTOVER STORAGE INSTRUCTIONS If stored in an airtight container at room temperature, homemade peanut brittle will stay fresh for 6-8 weeks. Do not refrigerate or the cool moisture will cause the brittle to soften.