Combine the sugar, syrup, butter, water, and salt in a large 3 quart saucepan. Cook over medium heat, stirring constantly, until sugar dissolves. Insert candy thermometer. Cook, without stirring to the soft crack stage, 270 degrees on the candy thermometer.

Remove from heat and add the peanuts. Return to heat and cook to 300 degrees, hard crack stage. Remove from heat. Stir in baking soda. Let the foaming syrup settle a bit, then quickly pour onto a well-buttered large cookie sheet. Stretch out as far as possible. Cool completely, then break into pieces. Store in an airtight container.

Nutless Variation: Use 2 cups of corn chex cereal instead of the peanuts, and increase the baking soda to 2 teaspoonfuls. Zowie! A "peanut" brittle that my allergic daughter can eat. Tastes almost like the real thing!