Go through popcorn and remove any unpopped kernels. Set aside. Lay a rectangle of parchment on the counter.
Heat a large pot (big enough for the popcorn) over medium low heat. Add butter and marshmallows. Stir until melted. Remove from heat and stir in kosher salt and vanilla extract. Add popcorn to pot and stir until completely coated. Allow to cool for 3 minutes.
Spray hands well with nonstick spray. Measure out a cup of the popcorn mixture. Roll into a ball. Set on prepared parchment. Repeat until you have used all the popcorn. Store in an airtight container.