Prep: Line a baking sheet with parchment or a silicone mat. Set aside.
Heat: In a heavy saucepan, combine sugar, corn syrup, and water. Stir over medium heat until sugar dissolves.
Boil: Stop stirring and let it boil until it reaches 300°F (hard crack stage), or until the mixture turns a golden amber color.
Add peanuts: Stir in the peanuts quickly and cook for about 1–2 more minutes, until golden.
Finish: Remove from heat and immediately stir in butter, vanilla, and baking soda. The mixture will foam up — stir fast!
Spread: Pour the hot brittle onto your prepared baking sheet and quickly spread it thin.
Cool: Let it cool completely (about 30 minutes), then break into pieces.