Heat oven to 200 degrees F. Grease 2 large cookie sheets with butter, keep warm in the oven.
(Keeping the cookie sheets warm allows the candy to spread to 1/4 inch thick without it setting up.)
Grease long spatula with butter, set aside.
In small bowl, mix baking soda, 1 teaspoon water, and the vanilla.
In 3 quart saucepan, mix sugar, 1 cup water and the corn syrup.
Cook over medium heat about 25 minutes, stirring occasionally, to 240 degrees 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.
Stir in butter and cashews. Cook about 13 minutes, stirring constantly, to 300 degrees F.
Immediately remove from heat.
Quickly stir in baking soda mixture until light and foamy.
Pour half of candy mixture onto each cookie sheet and quickly spread to about 1/4 inch thick with buttered spatula.
Cool completely at least 1 hour. Break into pieces. Store in airtight container at room temperature.