Combine sugar and syrup in a medium-size heavy saucepan; stir well. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches soft ball stage (234°). Remove from heat; stir in half-and-half and butter. Add cashews, stirring well. Serve warm over ice cream or plain cake. Store in an airtight container in refrigerator.