In 2-cup microwavable measuring cup, microwave brown sugar, butter and corn syrup uncovered on High 1 to 2 minutes, stirring after 1 minute, until melted and smooth. Stir in baking soda until dissolved. Pour over popcorn mixture, stirring gently until evenly coated. Microwave on High 3 minutes, stirring every minute. Spread on lined cookie sheet. Cool 10 minutes.