Spread a large sheet of parchment or wax paper over a baking sheet or clean counter. Place the popped popcorn in a large mixing bowl and pick out any unpopped kernels.

Add the white chocolate chips and coconut oil or shortening (if using) to a microwave-safe bowl. Microwave on 50% power for 30 seconds, then stir well. Continue heating in 20–30 second bursts at 50% power, stirring after each, until mostly melted and smooth. The last small bits will melt as you stir.

Stir in the vanilla extract (if using) and about half of the fine sea salt. Taste a tiny dab carefully and adjust the salt so the mixture is pleasantly sweet with a light salty background.

Drizzle about half of the melted white chocolate over the popcorn. Use a large spatula or clean hands to toss gently, scooping from the bottom so the popcorn begins to coat evenly. Drizzle on the remaining white chocolate and continue tossing until the popcorn is lightly and evenly coated without heavy clumps.

While the coating is still wet, sprinkle your desired mix-ins—such as mini M&M’s, sprinkles, crushed candy canes, chopped nuts, or mini marshmallows—over the popcorn. Toss gently once more so the toppings adhere to the white chocolate.

Transfer the coated popcorn to the prepared parchment-lined baking sheet. Spread into an even layer so it cools quickly and doesn’t set in a single mound. Let sit at room temperature for 20–30 minutes, or until the white chocolate is firm and dry to the touch. If the kitchen is warm, chill the tray in the refrigerator for about 10 minutes.

Once set, break the popcorn into large clusters. Taste and sprinkle with a little additional fine sea salt if you like a more pronounced salty-sweet flavor. Serve in bowls for snacking or package in bags, jars, or tins for gifting.

For a fancier presentation, drizzle a little extra melted white or dark chocolate over the cooled popcorn and finish with an extra pinch of sprinkles. Let set before serving or packaging.