Pop the popcorn on the stove. Then spray a large glass bowl with vegetable oil. Measure and place cooked popcorn in the prepared bowl.
Place butter in a large glass pyrex measuring cup, and melt in the microwave for about a minute. Make sure to watch the butter, so it doesn't overflow.
Mix corn syrup, brown sugar, and salt into the melted butter.
Microwave on high and cook for 4-5 minutes, stirring every 1 ½ minutes. Watch carefully so that it doesn't boil over the edges of the pyrex.
Once the caramel mixture is cooked, immediately add baking soda and vanilla, stir well. The mixture will foam up, but keep stirring.
Pour caramel over the popcorn and gently stir with a spoon.
I add the caramel mixture in stages and stir after each addition. Remove popcorn kernels by transferring the popcorn to another glass bowl. This is an optional step.
Microwave the popcorn mixture for 1 minute. Stir popcorn and cook for another 15-30 seconds if it needs the extra time. I have a high-powered microwave, and so my popcorn was done in 1 minute. Be careful not to overcook and burn the popcorn.
Remove popcorn from the bowl, and spread on parchment paper. Break up clumps of popcorn if you like.