Grease two large cookie sheets (with sides).

In a large 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. Dissolve the sugar by swirling the pot often over the burner.

Once steam begins to rise around the edges of the top, remove cover and reduce heat to medium. Insert candy thermometer, and continue to boil until temperature reaches 230 degrees (thread stage), about 1 minute.

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.

Immediately remove from heat, and quickly add butter, vanilla and baking soda. Stir with metal spoon until butter melts; mixture will become foamy. Pour half of mixture down center of each pan; spread evenly with spatula. Allow to cool for at least 1 hour.