PREP: Line an extra-large sheet pan with foil, parchment paper, or a silicone liner. Set aside.
MIX: In a very large bowl, measure and add the popped popcorn, making sure no kernels make it in! (See Note 2.) Break the pretzels (roughly in half), measure, and add those in. Add in the M&Ms.
WHITE CHOCOLATE: Place the white chocolate chips and oil in a microwave-safe bowl. Heat in bursts of 30 seconds stirring in between each burst until the chocolate is melted and smooth. Be patient to avoid burning the chocolate. Once the white chocolate is smooth and fully melted, use a spatula to scrape it out and over the popcorn mix. Working quickly, toss everything to get evenly coated in the white chocolate.
FINISH MIX: Spread mixture onto prepared pan, space in one layer, and immediately sprinkle with sprinkles (if using).
HARDEN: Let the popcorn harden at room temperature or in the fridge until it becomes solid. Once solid, break it into pieces and enjoy! Store any leftovers in an airtight container at room temperature. For the best flavor, eat within 1-2 days.
Notes
Note 1: Popcorn: Purchasing already popped popcorn makes this recipe come together a little bit quicker, but it’s a bit more spendy. If you’d rather pop your own, these are the packets we use (lightly salted plain popcorn). Of course, you can also use kernels in a popcorn machine or on the stovetop. Whatever you choose to use, measure out 8 cups of cooked popcorn for this recipe. If you use kernels, don't forget to add a touch of salt.Note 2: Remove the kernels: Put the popcorn in a large bowl and place a cooling rack over the top. Invert and shake over a plate....and all of the unpopped kernels will come out! Scoop the rest of the popcorn out of the bowl and proceed with the recipe.