Step 1

Generously coat a large marble slab or 2 rimmed baking sheets with unsalted butter.

Step 2

Place 2 cups granulated sugar, 1 cup light corn syrup, and 1/4 cup water in a large 3-quart heavy bottomed saucepan. Cook over medium heat (don't be tempted to cook at a higher heat as you risk scorching), stirring constantly with a heatproof spatula, until the sugar dissolves.

Step 3

Clip a candy thermometer onto the saucepan and continue cooking, stirring frequently to prevent scorching, until the mixture reaches 285°F (soft crack stage), about 15 minutes. Meanwhile, measure out 2 cups salted, roasted peanuts, 3 tablespoons unsalted butter, 2 teaspoons baking soda, and 1 teaspoon vanilla extract.

Step 4

Add the peanuts and unsalted butter to the saucepan and continue to cook, stirring constantly, until the mixture reaches 295°F. (This is just slightly above the soft crack stage, but not yet at the hard crack stage.) Immediately remove the saucepan from the heat and take off the candy thermometer.

Step 5

Working quickly, add the baking soda and vanilla extract to the saucepan and stir to combine. The mixture will foam. Immediately pour the mixture onto the prepared marble slab or baking sheets and spread it out into as thin a layer as possible with an offset or heatproof spatula.

Step 6

When the mixture begins to cool and harden, after about 10 minutes, run a long, thin metal spatula or butter knife underneath the brittle to gently loosen it from the surface. This will help it cool and help prevent sticking. Once completely hardened and cooled, break it into pieces.