Put 8 cups popped popcorn into a large bowl. If using the optional 1 cup salted mixed nuts, add them now and toss to combine.
Place 1 cup butterscotch chips in a microwave-safe bowl. Microwave in 30-second increments, stirring after each increment, until the chips are completely melted and smooth.
Stir 1 teaspoon coconut oil into the melted butterscotch until fully incorporated and smooth.
Pour the melted butterscotch evenly over the popcorn (and nuts). Gently stir or fold with a spatula until the popcorn is evenly coated.
Spread the coated popcorn in an even layer on a baking sheet lined with parchment paper, waxed paper, or a silicone baking mat.
Place 1/4 cup white chocolate chips in a small microwave-safe bowl. Microwave in 15–30 second increments, stirring after each increment, until the white chocolate is melted and smooth.
Drizzle the melted white chocolate over the popcorn using a spoon or fork.
Allow the popcorn to cool completely at room temperature until the coatings are set. To speed cooling, place the baking sheet in the refrigerator for a short time.
Once set, break the popcorn into chunks and serve.