Coat 2 rimmed baking sheets with vegetable oil spray; set aside.
In a heavy 5-quart saucepan with a tight-fitting lid, combine sugar, corn syrup, and 1/2 cup water. Cover the saucepan, and bring to a boil over high heat, about 5 minutes, Dissolve the sugar by swirling the pot often over the burner. Be sure to brush down the sides of the saucepan with cold water to prevent crystals from forming.
Once the sugar is dissolved, reduce the heat to medium, and insert a candy thermometer, and continue to cook until the temperature reaches 230 degrees F.
Add the peanuts, and stir constantly with a metal spoon until the mixture reaches 300 degrees F, about 15 minutes. At this point the mixture should be a rich, golden color.
Immediately remove from heat, and quickly add butter, vanilla, and baking soda. Stir well with a metal spoon until butter is melted; mixture will become foamy. Pour half of the mixture down the center of each pan, spread it evenly with a spatula. Allow to cool for at least 1 hour.
Turn brittle out of baking sheets, and snap into shards. Peanut brittle can be stored in an airtight container for several weeks.