Melt in microwave on 100% power:

1/3 c butter (or oleo)
I suggest you put something over the bowl to keep the butter from splattering.

Add:

2/3 c brown sugar
1/3 c light Karo syrup

Microwave on 100% power until mixture boils, about 3 minutes. Stir once.

Continue to microwave at the boiling point (100% power) for 3 more minutes. Did I mention this should be done in a microwave-safe bowl?

Add:

1/4 t baking soda
1/2 t vanilla

Add and mix well:

2 quarts of popped popcorn

Return to microwave and heat at 75% power for about 6 minutes, or until popcorn is well coated. Stir every 2 minutes to help this along.

Cool and break into pieces. Store tightly-covered if you have any left.