Preheat the oven to 350 degrees.Line two baking sheets with parchment paper.
In a medium bowl, mix together the flour, baking soda, cornstarch, and salt together. Set aside.
In a mixer, beat the butter and both sugars together until light and fluffy. Add in the egg and yolk, then the vanilla.
Slowly add the flour mixture into the wet ingredients and mix until just combined.
Fold in the popcorn. It will seem as though it is an impossible task, but the popcorn will break down and incorporate into the dough. Fold in the remaining ingredients until incorporated.
Measure out the dough into one tablespoon balls and place on the baking sheet about an inch apart. Bake for about 8 – 9 minutes, or until a light golden brown.