Butter large baking sheet; set aside. Combine brown sugar and syrup in 2 quart heavy saucepan. Cook over medium heat, stirring occasionally, until sugar is dissolved and mixture comes to a full boil [4 to 9 minutes].
Add butter; continue cooking, stirring occasionally, until candy thermometer reaches 300 or small amount of mixture dropped into ice water forms a hard brittle strand [10 to 18 minutes]. Remove from heat; stir in cashews and baking soda. Immediately pour mixture onto prepared baking sheet, spreading evenly to 1/8 to 1/4 inch thickness. Cool completely. Break into pieces. Store in container with tight-fitting lid.