Place popcorn in a large brown paper sack. (Like the kind you get from the grocery store.) Set aside.
In a large microwave safe casserole dish (read: as large as will fit in your microwave), melt the butter. Stir in brown sugar, corn syrup and salt. Cover and microwave on high for 2 minutes. Be careful the mixture will be very hot. Sir in baking soda. The mixture will foam up. Microwave on high for 1 and 1/2 minutes.
Pour over the popcorn in the paper sack and shake well. Roll down the top of the paper sack and place the whole thing in the microwave. Turn of the turntable and microwave on high for 1 and 1/2 minutes. Shake again. Dump the popcorn from the bag onto a cookie sheet to cool.