Preheat oven to 350 degrees. Line baking sheets with parchment paper or baking mats, set aside.
Place butter in a microwave safe bowl and heat for 30-45 seconds (full strength) or until it's almost completely melted but has a few soft pieces left.
In a large mixing bowl, whisk together flour, baking soda, cornstarch, and salt.
In the bowl of a stand mixer (or a large mixing bowl with a hand mixer), beat together butter, sugars and vanilla for 1 minute. Add the eggs one at a time, mixing in between additions. Turn mixing speed to low and gradually add in the dry ingredients. Beat until just combined or until a soft dough forms. Fold in the popcorn, chocolate chips, M&Ms, and pretzel bits. Now you can either bake the dough straight away or chill it for 2-3 hours for thicker, chewier cookies.
To bake the cookies, using a standard size cookie scoop (approximately 1.5 tbsp.) to form balls of dough. Place onto the prepared sheets, leaving at least 2 inches in between.
Place in the oven and bake for 10-12 minutes, rotating the sheet half way through the baking process. Remove from oven when cookies are golden brown in color around the edges. Let cookies cool on pan for 3-4 minutes, then transfer to a wire rack to cool completely.
Notes
-Try adding toffee bits or caramel corn instead of kettle corn to these cookies!