
Lightly grease a large baking sheet (13x18”) with butter. Line it with parchment paper and lightly butter the paper. Make sure all of your ingredients are measured out and ready to go before you start.
In a medium, heavy-bottomed saucepan with tall sides (at least 4-quarts), stir gently to combine the sugar, corn syrup, water, and salt. Scrape the sides of the pot with a rubber spatula to clean any sugar off the sides. Clip a candy thermometer on the side of the pot (making sure that it doesn’t touch the bottom). Place the pan over medium heat.
Bring the mixture to a boil, gently stirring occasionally until the sugar is fully dissolved, for 6 to 10 minutes, until it reaches 250°F.
Immediately add the butter and stir constantly until it melts. Let the mixture return to a boil.
Cook for another 12 to 15 minutes, or until the mixture reaches 280°F, then stir in the peanuts. Cook, stirring slowly but consistently, until the temperature reaches 305°F.
Remove the pan from heat and immediately add the vanilla and baking soda. Stir well to combine. (The mixture will bubble up quite a bit, so don’t be alarmed.) Immediately pour the mixture onto the prepared baking sheet and quickly spread it out using a spatula. The brittle should completely cover the sheet pan and be thinner than the peanuts embedded in it.
Allow the brittle to cool completely, about 45 minutes, before breaking into pieces.