Step 1

Preheat oven to 250F.

Step 2

Heat 3 tablespoons of grapeseed oil in the stovetop popcorn popper to 400F. If you don't have an IR thermometer, just throw in three kernels together with the oil and wait for all three to pop. Once they do, the popper is ready for popping.

Step 3

Throw in 1/2 a cup of mushroom popcorn kernels, close the lid and start agitation.

Step 4

Once the popping sounds stop, remove the popcorn popper from heat and dump the popped popcorn into a large bowl.

Step 5

Make sure to put the popcorn in a fairly large bowl. You want the popcorn to not fill more than 2/3 of the bowl. Half way is ideal. Otherwise it will be very hard to stir it without some popcorn spilling out. If you don't have one of the required size, do it in two batches.

Step 6

In a medium saucepan over medium heat, melt the butter. Stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly. Boil without stirring for 4 minutes. Remove from heat and stir in soda and vanilla.

Step 7

Pour over popcorn in a thin stream, stirring to coat.

Step 8

Place the coated popcorn in one large (12 x 18 inch) baking pan lined with parchment paper. If you don't have one, use two smaller pans.

Step 9

Transfer to the oven and bake, stirring every 15 minutes, for 1 hour.

Step 10

Remove from the oven and let cool for a few minutes until still very warm but manageable to handle. At this point start quickly separating the popcorn into individual pieces and spreading them out. It helps to do it on a large table so you have enough space. Don't let the popcorn cool completely before doing this otherwise the pieces will stick together and will break when you try to separate them.

Step 11

Store in an airtight container for up to a couple of weeks.