Heat oven to 200º. Generously butter large cookie sheet; keep warm in oven.
Mix brown sugar and maple syrup in 8-cup microwavable measure. Microwave uncovered on High 5 minutes. Stir in nuts.
Microwave uncovered on High 5 to 7 minutes or until syrup is bubbling and nuts are toasted--syrup will be very hot. Stir in butter.
Microwave uncovered 1 minute. Quickly and thoroughly stir in baking soda until mixture is light and foamy. Pour onto cookie sheet; quickly spread candy.
Cool 30 to 60 minutes or until hardened. Break into pieces. Store candy in airtight container at room temperature up to 2 weeks.