Grease two baking sheets and keep warm in a 200° oven. In a large saucepan, combine the sugar, corn syrup and 1 cup water. Cook over medium heat, stirring constantly, until a candy thermometer reaches 240° (soft-ball stage). Stir in butter, salt and peanuts. Continue heating, stirring constantly, until the mixture reaches 300°.
Meanwhile, combine the baking soda, 1 teaspoon water and vanilla. Remove saucepan from the heat. Stir in baking soda mixture. Quickly pour half the mixture over each baking sheet. Spread with a buttered metal spatula to a 1/4-in. thickness. Cool. Break into pieces. Store in an airtight container. Yield: about 2-1/2 pounds.