Pop the popcorn if making it fresh. Measure 8 cups and gently shake or pick out any unpopped kernels so no one bites into them. Add the popcorn to a very large mixing bowl.
8 cups popped popcorn
Add the mini pretzels and cereal to the bowl with the popcorn. Use clean hands or a large spoon to gently toss until somewhat evenly combined to form the snack mix base.
2 cups mini pretzels, 2 cups plain cereal
Add the candy corn, Halloween-colored chocolate candies, mini marshmallows, and peanuts or seeds (if using) to the bowl. Toss gently to distribute. Reserve the candy eyeballs for later so they stay visible on top.
1 cup candy corn, 1 cup Halloween-colored chocolate candies, 1/2 cup mini marshmallows, 1/2 cup roasted peanuts or candy-coated peanuts
Line a large baking sheet (or two smaller baking sheets) with parchment paper or wax paper to prevent sticking and make clean-up easy.
Place the white chocolate chips or candy melts in a microwave-safe bowl. Microwave on 50% power for 30-second intervals, stirring well after each, until mostly melted. Add 1 teaspoon of neutral oil, stir until smooth, and let the residual heat finish melting. If the mixture is too thick to drizzle, add up to 1 more teaspoon of oil and stir again.
12 oz white chocolate chips or candy melts, 1-2 teaspoons neutral oil
Pour the popcorn and candy mixture onto the lined baking sheet and spread it into an even, slightly crowded layer. Avoid piling it too thickly so the drizzle can coat more pieces.
8 cups popped popcorn, 2 cups mini pretzels, 2 cups plain cereal, 1 cup candy corn, 1 cup Halloween-colored chocolate candies, 1/2 cup mini marshmallows, 1/2 cup roasted peanuts or candy-coated peanuts
Using a spoon, fork, or a small squeeze bottle, drizzle the melted white chocolate evenly over the entire tray of snack mix in zigzag motions. Aim to hit as many pieces as possible; the chocolate acts as a light glue and sweet coating but does not need to fully cover everything.
12 oz white chocolate chips or candy melts, 1-2 teaspoons neutral oil
While the white chocolate is still wet, scatter the candy eyeballs and Halloween sprinkles evenly over the top. Gently press a few eyeballs down so they adhere. If using flaky sea salt, lightly sprinkle a pinch over the mix for extra sweet-and-salty flavor.
1/4-1/3 cup candy eyeballs, 2-4 tablespoons Halloween sprinkles, pinch flaky sea salt
Let the tray sit at room temperature until the chocolate is fully set and firm to the touch, about 20–30 minutes. If your kitchen is warm, you can place the tray in the refrigerator for about 10–15 minutes to speed things up, but do not leave it too long so the popcorn doesn’t absorb moisture.
Once the chocolate has set, break the monster munch into large clusters. Transfer to a serving bowl or divide into individual treat bags for parties or gifts. Serve at room temperature.