Have a large paper bag handy, spread some wax paper on your table, grab your ingredients and here we go!
Melt butter with brown sugar and corn syrup in a 2 quart glass bowl in the microwave for about 1 minute. Stirl well and continue cooking the caramel to 310-320 degrees. This will take about 3 minutes (depending on your microwave) - stir every minute to blend the butter in evenly. The caramel must get to hard crack stage. I have a microwave thermometer that I use to test the temperature., but you can also test it by dripping a bit of the caramel into a cup of cold water.
Add in the baking soda and stir - this will cause the caramel to expand and get fluffy ! This may be what makes it addicting.
Quickly spray your paper bag with cooking spray, dump your puffcorn into the bag followed by the caramel, and shake like mad! I use oven mitts during this stage to keep my hands from getting burned. Place the bag in the microwave for 1 minute. Shake well. Another minute in the microwave, shake well. A final minute and shake well! Feel free to dance while shaking the bag.
Spread the puffcorn out over the waxed paper and let cool completely. When it is cool, break it apart if needed and it's ready to eat. Did I mention it's addicting? Don't say I didn't warn you!