1. Coat two 12-by-17-inch baking pans (with low sides) with vegetable-oil spray.
2. In a heavy 5-quart saucepan with a tight-fitting lid, combine sugar, corn syrup, and 1/2 cup water. Cover the pot, and bring the mixture to a boil over high heat, about 5 minutes. Swirl the pot often over the burner to dissolve the sugar. Keeping the lid on will prevent sugar crystals from forming inside the pot; however, should crystals form, wash down the sides with a wet pastry brush.
3. Once steam begins to rise around the lid, remove it and reduce heat to medium. Insert candy thermometer, and continue to boil until temperature reaches 230 degrees (thread stage), about 1 minute.
4. Add the peanuts, and stir constantly with a metal spoon until the mixture reaches 300 degrees (hard-crack stage), about 13 to 18 minutes. At this point, the mixture should be a rich, golden brown.
5. Immediately remove from heat, and quickly add butter, vanilla, and baking soda. Stir with a metal spoon until butter melts; mixture will become foamy. Pour half of mixture down the center of each pan; spread it evenly with a spatula. Allow to cool for at least 1 hour.
6. Turn brittle out of the pan, and snap it into shards. Peanut brittle can be stored in an airtight container for several weeks.