In a 1 1/2 quart microwave dish, stir together brown sugar, butter, or margarine, corn syrup and salt. Microwave on high 2 minutes; stir. Microwave on high for 2 more minutes. Stir in baking soda. Pour mixture over popped corn in large brown paper grocery bag. Shake well to coat. Place bag in microwave for 1 1/2 additional minutes. Pour in Butterfinger and shake to coat again. Pour onto waxed paper to cool.