In another microwave-safe bowl, combine the brown sugar, butter, corn syrup and salt. Cover and microwave on high for 1 minute; stir.
Microwave 2 minutes longer.
Stir in vanilla and baking soda. Pour over popcorn mixture. Microwave, uncovered, on high for 3 minutes, stirring several times.
Spread on greased baking sheets to cool. Store in an airtight container.