Buttered popcorn and brown sugar are a perfect match in this chewy blondies recipe that’s perfect for those who love their desserts with a hint of salt. Use your favorite brand of microwave popcorn for this recipe—the saltier and buttery-er, the better! Just be sure to take a minute to pluck out any unpopped kernels before you mix them into the batter (your dentist will thank you).
Preheat the oven to 350 degrees F. Line a 9-by-13-inch baking pan with aluminum foil, leaving a 2-inch overhang on the long sides. Grease the sides with butter.
Microwave the popcorn according to the package directions. Allow to cool, then discard any unpopped kernels.
Combine the melted butter, brown sugar, eggs and vanilla in the bowl of a stand mixer fitted with the paddle attachment and beat until creamy and evenly combined. Add the kosher salt and flour and continue to mix; you should have a creamy, loose batter. Fold in the popcorn, a few cups at a time, with a rubber spatula until it's fully coated in the batter. (Make sure there are no unpopped kernels in the batter—you don't want to break a tooth!)
Spread the batter out in the prepared pan (it will be thick) and use fingers dipped in cold water to pat it into an even layer. Sprinkle with a bit of flaky sea salt.
Bake until the edges darken a little bit and the top is set and glossy, 18 to 20 minutes. Remove from the oven and let cool in the pan for 10 minutes before lifting out with the foil overhang onto a wire rack to cool completely. (Or refrigerate until cool.)
Cut into 16 squares and serve or store at room temperature in an airtight container for up to 3 days.
If you prefer to make your own popcorn, use 1/4 cup kernels, popped, then butter and salt the popcorn. Be sure to sift through and discard any unpopped kernels!