Preheat oven to 225 degrees. Pop popcorn in the microwave for 2 minutes and 20 seconds (This is not the entire time the directions say, but that is what you want.) Pour into a large parchment lined roaster or 2 large parchment lined cookie sheets. You will have some unpopped kernels, just remove them. In saucepan, melt butter, add brown sugar and corn syrup. Boil exactly 5 minutes. Remove from heat and add vanilla and baking soda. Stir until well mixed. Pour mixture over popcorn and mix well, coating the popcorn. Bake for 1 hour, stirring every 15 minutes (flip it over, it will be sticky). Now, on the cooking, check often during the last 15 minutes of cooking. Don't let it cook too long, I have removed it before the entire hour is complete due to the fact that my oven is a self-cleaning oven and cooks hotter that some ovens, so I have to be careful when baking, but 1 hour is what the directions say, so that is how I've stated it. Pour out onto waxed paper and break apart. Store in an airtight container.