Step 1: Prepare a large baking sheet by buttering it generously or lining it with a silicone mat. Gather all your ingredients and have them measured and ready, as the process moves quickly.
Step 2: In a heavy-bottomed 3-quart saucepan, combine the granulated sugar, light corn syrup, and water. Stir gently over medium heat until the sugar dissolves completely.
Step 3: Attach a candy thermometer to the side of the pan, ensuring the bulb does not touch the bottom. Bring the mixture to a boil without stirring. If any sugar crystals form on the sides of the pan, wipe them down with a wet pastry brush.
Step 4: Continue to boil the sugar mixture until it reaches 295°F (146°C). This is just shy of the hard crack stage.
Step 5: Immediately stir in the warm roasted peanuts. Continue cooking and stirring constantly until the mixture reaches 305°F (152°C) (hard crack stage).
Step 6: Remove the pan from the heat. Quickly stir in the butter until melted and fully incorporated.
Step 7: Immediately sprinkle in the baking soda and vanilla extract. Stir vigorously and quickly for about 10-15 seconds until the mixture becomes light, foamy, and opaque. Do not overmix.
Step 8: As soon as the baking soda is mixed in, immediately pour the hot brittle mixture onto your prepared baking sheet. Working quickly, use a heat-safe spatula or the back of a buttered spoon to spread the brittle thinly and evenly. The thinner it is, the crispier it will be.
Step 9: Let the brittle cool completely at room temperature (1-2 hours) until it is hard and firm. Do not refrigerate, as this can make it sticky.
Step 10: Once completely cooled, break the brittle into irregular pieces. Store in an airtight container at room temperature for several weeks.