Butter two large baking sheets; set aside. Butter the sides of a 3-quart heavy saucepan. In saucepan combine sugar, corn syrup, the water, and 1/4 cup butter. Cook and stir over medium-high heat until mixture boils. Clip a candy thermometer to side of pan. Reduce heat to medium-low; continue boiling at a moderate, steady rate, stirring occasionally, until thermometer registers 275 degrees F, soft-crack stage (about 30 minutes). Adjust heat as necessary to maintain a steady boil.
Stir in cashews; continue cooking over medium-low heat, stirring frequently, until thermometer registers 295 degrees F, hard-crack stage (15 to 20 minutes more).
Remove saucepan from heat; remove thermometer. Quickly sprinkle baking soda over corn syrup mixture, stirring constantly. Immediately pour onto the prepared baking sheets. Using two forks, lift and pull candy as it cools. Cool completely; break into pieces. Store in an airtight container at room temperature for up to 1 week.