Unsalted butter, room temperature, for baking sheet
1 1/2 cups sugar
1/2 cup light corn syrup
3/4 cup cold water
Pinch of table salt
2 1/2 cups dry-roasted peanuts
1 teaspoon pure vanilla extract
1 teaspoon baking soda
Vegetable oil, for spatula
Brush a 9-by-13-inch rimmed baking sheet with butter; set aside. Combine the sugar, corn syrup, water, and salt in a medium saucepan.
Bring to a boil over medium-high heat, stirring until the sugar has dissolved. Using a pastry brush dipped in water, brush away any sugar crystals on the side of the pan to prevent recrystallization.
Cook, swirling occasionally, until mixture reaches the softball stage on a candy thermometer (238 degrees). Stir in the nuts; continue to cook, stirring often so the nuts do not burn, until the mixture is golden amber in color.
Carefully stir in the vanilla and baking soda. The mixture will foam up in the pan.
Pour the mixture onto the prepared baking sheet, and quickly spread into a 1/2-inch-thick layer with an oiled metal spatula. Set the tray aside until completely cool.
Break brittle into pieces; store in an airtight container at room temperature up to 1 month.