Lightly and evenly grease a jelly roll pan with butter and set aside (keep nearby).
Measure out your butter, peanuts, baking soda, and vanilla extract. Set these aside but keep them nearby. It’s important to have these ready before you begin as once your candy reaches the necessary temperature you will need to move quickly.
Combine sugar, water, corn syrup, and salt in a medium-sized saucepan and stir occasionally, using a damp pastry brush to wipe any sugar crystals from the side of the pot, over medium heat. Continue to cook until sugar is dissolved and mixture comes to a boil. This will take some time, don't rush the process by turning up your heat or you will ruin your brittle.
Once mixture is boiling, attach your candy thermometer (make sure the bottom of the thermometer isn’t touching the bottom of the pot!) and stir occasionally until temperature reaches 250°F (121°C).
Add peanuts and stir continuously until mixture reaches 305°F (151°C)
Remove from heat and immediately add butter, baking soda, and vanilla. Stir well until butter is melted and mixture is well-combined.
Pour into prepared jelly roll pan and use 2 forks to pull the brittle apart and to help spread evenly (careful the pan gets very hot!).
Allow to cool for at least an hour before breaking into pieces and enjoying.
Store peanut brittle in an airtight container (for best results use wax or parchment paper to divide pieces as they may stick together) in a dry place, but not in the refrigerator. Peanut Brittle will keep up to 2 months.