Measure out peanuts, butter, baking soda and vanilla and have them ready.
Add sugar, and water to a medium saucepan and stir well. Stir in corn syrup. Cook mixture over medium heat (don’t be tempted to turn the heat up!), stirring occasionally, until it comes to a gentle boil.
Attach your candy thermometer to the edge of the pot, and make sure it is submerged in the liquid, but not touching the bottom of the pot. Cook, stirring occasionally until temperature reaches 250°F (121°C).
Add the peanuts and stir the mixture constantly until the candy thermometer temperature reaches 300 degrees F.
Remove mixture from heat and immediately stir in butter, baking soda and vanilla. The mixture will foam and change texture.
Carefully pour the hot mixture onto the prepared pan and use a knife or spoon to quickly (and carefully!) spread the mixture into an even layer.
Allow to cool completely (at least 30 minute) before breaking apart and eating.
Store the completely cooled peanut brittle in an airtight container.
To make ahead: Peanut brittle can be made several days or even weeks in advance. It stays fresh at room temperature for 6-8 weeks weeks.To store: Store peanut brittle (once it's completely cooled) in an airtight container at room temperature. Do not refrigerate as the moisture from the fridge will cause the brittle to soften. Store for 6-8 weeks.To freeze: Peanut brittle can be frozen and stored up to 3 months. Allow the brittle to cool completely and store it in a freezer safe, airtight container.