Line a large cookie sheet with parchment paper. Set aside.
Pour popcorn into a large bowl, should be twice the size of the amount of popcorn you are using. I use a small roasting pan quite often for mixing candy popcorns. Set this aside.
Place a large saucepan on the stove, place the butter in the saucepan followed by the brown sugar. Turn heating element on medium heat and cook the mixture until the sugar dissolves. This should take about 3 to 5 minutes, more or less depending on your heat level and pan.
Add the root beer and cook over medium heat for about 3 minutes, bringing it to almost a boil. You want it bubbling a bit across the surface.
Add the corn syrup and allow to cook over medium-high heat until it begins to thicken. It should bubble up throughout the process, almost like making fudge, it will start to bubble and thicken after a few minutes. Stir every so often to make sure you don't heave the heat too high. You don't want the mixture to stick to the bottom of the pan.
Once it begins to thicken considerably, and starts to resemble caramel (takes about 5 to 8 minutes, or so) you can take it off the heat, turn off the element, and add the salt, and vanilla. You can also add the root beer extract if you're using it. It just gives a stronger root beer flavour. I did not use it for this batch of popcorn, but do use it on occasion depending on how strong I want the flavour to be. Stir the mixture until smooth. Allow to cool for about 5 to 7 minutes so when you pour it over the popcorn that the popcorn won't shrivel up.
Once it's cool you can pour it over the popcorn, then use a big spoon to mix everything up really well. You want to coat all the popcorn thoroughly. Once it's even coated transfer it to the cookie sheet you prepared earlier. Spread it out evenly on the pan. Allow to cool on the pan until the root beer mixture hardens up. Drying takes about 1 to 2 hours. Don't put it in the oven or you'll just melt the mixture again!
You can stir it every 30 minutes so it doesn't stick together. Personally I like it to stick together a little bit so I leave mine in chunky pieces.
Once dry you can serve it or place it in an airtight container. Best if eaten on day 1, but will keep for 2 to 3 days. If it lasts that long.
Serve with a bit old root beer loving smile!
This recipe can be adjusted if you like less candy coating on your popcorn. Use 10 to 12 cups of popcorn instead of 8 if you like yours coated lightly.