Line a large baking sheet with parchment paper or a silicone mat. Set aside. Have a very large mixing bowl ready for tossing the popcorn.
Add 10 cups of popped popcorn to a very large bowl. Pick out and discard any unpopped kernels. Sprinkle the mini marshmallows over the popcorn and gently toss to distribute.
10 cups popped popcorn, 1 cup mini marshmallows
In a small bowl, whisk together the hot cocoa mix, powdered sugar, and fine sea salt until well combined. Set aside.
1/4 cup hot cocoa mix, 2 tablespoons powdered sugar, 1/4 teaspoon fine sea salt
In a microwave-safe bowl, combine the semisweet chocolate chips, milk chocolate chips, and butter. Microwave on 50% power in 20–30 second bursts, stirring after each, until smooth and fully melted, 1–1 1/2 minutes total. Do not overheat.
1 cup semisweet chocolate chips, 1/2 cup milk chocolate chips, 3 tablespoons unsalted butter
Stir the vanilla extract into the melted chocolate mixture until evenly incorporated. If it thickens too much, warm the chocolate for another 10 seconds at 50% power and stir again.
1 teaspoon vanilla extract
Drizzle about half of the warm chocolate mixture over the popcorn and marshmallows. Use a large spatula or clean hands to gently toss, reaching down to the bottom of the bowl. Add the remaining chocolate and continue tossing until most of the popcorn is coated. Some white spots are fine.
10 cups popped popcorn, 1 cup mini marshmallows, 1 cup semisweet chocolate chips, 1/2 cup milk chocolate chips, 3 tablespoons unsalted butter, 1 teaspoon vanilla extract
While the chocolate is still soft and glossy, sprinkle the hot cocoa–powdered sugar–salt mixture all over the coated popcorn. Toss again, turning the popcorn from the bottom and rotating the bowl to get a light, even dusting that clings to the chocolate.
1/4 cup hot cocoa mix, 2 tablespoons powdered sugar, 1/4 teaspoon fine sea salt
Spread the coated popcorn mixture onto the prepared baking sheet in an even layer. If desired, sprinkle with crushed peppermint candies and extra mini chocolate chips, and/or drizzle with melted white chocolate.
1 to 2 tablespoons crushed peppermint candies or candy canes, mini chocolate chips, 2 to 3 tablespoons white chocolate chips
Let the popcorn sit at room temperature for 20–30 minutes, or until the chocolate is set and dry to the touch.
Once set, gently break the hot cocoa popcorn into clusters. Transfer to a serving bowl and enjoy immediately, or pack into treat bags or airtight containers for gifting or storage.