2cupsroasted & salted peanuts*Make sure to use a brand that does NOT still have the skins on the peanuts. They may be labeled as “blanched”.*
Line a large rimmed baking sheet with a silicone mat. Set aside.
In a large stockpot, fitted with a candy thermometer, bring to a boil the water, granulated sugar, light corn syrup, and unsalted butter. This should take anywhere from 12-16 minutes. DO NOT DISTURB OR STIR THE COOKING SUGARS UNTIL YOU START TO SEE IT GETTING LIGHTLY GOLDEN IN COLOR. You want your brittle mixture to reach 300°F (Hard crack stage) on your candy thermometer. Turn off the heat immediately. If you have an electric stove, carefully move the pot to a cool burner so that your sugar mixture does not keep cooking.
Working very quickly, stir in the baking soda, vanilla extract, and roasted peanuts. Be very careful as the baking soda will cause the hot brittle mixture to bubble up rapidly.
Immediately pour your hot brittle mixture onto your prepared baking sheet and quickly spread out the peanut brittle mixture into as much of an even layer as possible. You should get an approximate 9x12 inch rectangle. You can use a plastic spatula to push, and spread, out the thick mixture. It will start to harden very quickly so you have to work fast.
Allow your peanut brittle to cool on the counter completely before breaking it into pieces.