Preheat oven to lowest temperature possible and generously grease a rimmed baking sheet with cooking spray.
Place baking sheet to warm in the oven while you make your brittle.
Heat a small metal saucepan over low heat - keep this warm while you make the brittle, you will use this to help press out the brittle.
In a medium sized saucepan, place brown sugar, granulated sugar, corn syrup, salt, and water.
Attach a candy thermometer to the side of the pan, then over medium heat, bring the mixture to 300 degrees F stirring frequently (this takes about 30 minutes).
Remove from heat and quickly stir in butter, vanilla, baking soda, then peanuts.
Immediately pour the brittle onto the warm prepared baking sheet.
Grease the bottom of the warmed small metal saucepan generously with cooking spray, then carefully press out the mixture to 1/4 inch - 1/2 inch thick (the brittle, sheet pan, and saucepan are VERY hot).
Let cool slightly at room temperature.
Clean the dirty saucepans and candy thermometer immediately with hot water and soap, so the brittle doesn't solidify.
Place the brittle in the freezer to cool completely.
Snap into pieces, place into an airtight container, then store in the freezer.
Enjoy frozen, let chill in the fridge, or enjoy at room temperature.