Pull out your air popper, pop ½ cup of popcorn kernels the old-fashioned way and set it aside in a large bowl.
Measure the butter, brown sugar, and corn syrup into a medium pot and place it over medium heat on the stovetop. (Please note that I was making a double batch, so picture 2 shows twice the amount of butter and brown sugar called for in the recipe)
Once the butter is melted, the contents in the pot are mixed together and begin to boil; turn down the heat to medium-low heat and boil the caramel for 3 minutes. This will slightly thicken the sauce while the sugar dissolves.
Remove the pot from the heat and immediately whisk in half a can of sweetened condensed milk.
Pour the hot, gooey caramel sauce over the popcorn and mix until all the popcorn pieces are well coated.
EXTRAS - If you're planning to add mix-ins like seasonal sprinkles, a cup of M&M candies, or peanuts, this is the time to add them.
TIP - It is helpful to use a rubber spatula or a silicone spatula for this instead of a wooden spoon for easier cleaning.
Spray the inside of a 1 cup measuring cup with non-stick cooking spray.
Scoop 8 cupfuls of caramel popcorn onto a sheet of wax paper, spacing the piles out so they don't touch.
TIP - as the caramel is very moist, parchment paper doesn't work nearly as well as wax paper because it gets a little soggy.
If you have popcorn left in the bowl, divide it up among the 8 piles.
Spray your hands lightly with non-stick cooking spray (this keeps the caramel from sticking to your hands) and press each pile into a ball.
The popcorn ball tends to re-expand a bit with the first pressing. I usually go back and re-press them into tighter balls after I've shaped them all the first time.
The balls of caramel popcorn should be roughly 3-inch balls (a little smaller than a softball).
Allow them to set and cool completely before wrapping each ball in plastic wrap, putting them in individual holiday bags, or simply placing them all in an airtight container.