To finish in the microwave: Place the butter, brown sugar, salt and syrup in a microwave safe dish. Microwave for 2 minutes and stir. Microwave for 30 seconds and stir. Microwave for 30 seconds more and stir in the baking soda. Pour the mixture over the popcorn in the bag. Shake the bag to evenly distribute the mixture over the popcorn.
To finish in the oven: Place the butter, sugar, salt and syrup in a pan and cook over medium heat until the butter has melted and bubbles form around the edge. Take off the heat and add the backing soda. Pour over the popcorn in the bag. Shake well to evenly distribute the mixture over the popcorn. Pour onto a baking sheet and cook for 1 hour in a 200 degree oven, stirring every 15 minutes.
