1Heat oven to 200°F. Grease 2 large cookie sheets with butter; keep warm in oven. (Keeping the cookie sheets warm allows the candy to be spread 1/4 inch thick without it setting up.) Grease long metal spatula with butter; set aside.
2In small bowl, mix baking soda, 1 teaspoon water and the vanilla; set aside. In 3-quart saucepan, mix sugar, 1 cup water and the corn syrup. Cook over medium heat about 25 minutes, stirring occasionally, to 240°F on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a soft ball that flattens when removed from water.
3Stir in butter and cashews. Cook about 13 minutes, stirring constantly, to 300°F or until small amount of mixture dropped into cup of very cold water separates into hard, brittle threads. (Watch carefully so mixture does not burn.) Immediately remove from heat. Quickly stir in baking soda mixture until light and foamy.
4Pour half of candy mixture onto each cookie sheet and quickly spread about 1/4 inch thick with buttered spatula. Cool completely, at least 1 hour. Break into pieces. Store in airtight container at room temperature.