Generously butter a rimmed baking sheet or line it with a silicone baking mat. Measure the butter, vanilla and baking soda into small dishes and set them next to the stove. The last step moves fast.
2 tablespoons butter, 1 teaspoon vanilla extract, 1 teaspoon baking soda
In a heavy-bottomed 3-quart saucepan, stir together the sugar, corn syrup, water and salt over medium heat until the sugar dissolves and the mixture comes to a boil. Clip on a candy thermometer, making sure the tip does not touch the bottom of the pan.
1 cup granulated sugar, 1/2 cup light corn syrup, 1/4 cup water, 1/2 teaspoon salt
When the syrup reaches 250°F, stir in the raw peanuts.
1 1/2 cups raw Spanish peanuts
Keep cooking, stirring often so the peanuts do not scorch, until the thermometer reads 300°F and the syrup is a light golden amber, about 8 to 12 minutes more.
Remove the pan from the heat. Quickly stir in the butter and vanilla until the butter melts, then stir in the baking soda. The mixture will foam up and turn lighter in color.
2 tablespoons butter, 1 teaspoon vanilla extract, 1 teaspoon baking soda
Immediately pour the mixture onto the prepared baking sheet and spread it as thin as you can with a buttered spatula.
Let the brittle cool completely, about 45 minutes, then break it into pieces.