Line a baking sheet with parchment paper. Pop the popcorn and chop up the chocolate if needed.
Spread the popcorn over the baking sheet, then sprinkle the chopped walnuts over the popcorn.
Melt the white chocolate in a glass bowl or tempered glass measuring cup in the microwave in 10-second intervals, stirring between intervals, until melted and smooth.
Drizzle the white chocolate over the popcorn & walnuts, then use a spoon to gently toss everything together.
Sprinkle the marshmallows and chocolate chips over the mixture, then toss again to combine.
Notes
I used microwaveable popcorn and popped it myself in advance. The buttery/salty flavor is a delicious contrast to the sweetness of the white chocolate. You can use pre-popped, bagged popcorn if you prefer to save time.
If you like, you can substitute pecans, peanuts, or shelled pistachios for the walnuts.
I used mini semi-sweet chocolate chips - feel free to replace with standard sized semi-sweet chips, or dark chocolate chips.
Want to bulk up this snack mix or add more texture/flavor? Consider adding a dry breakfast cereal to the mix. Some good options are Chex squares, Golden Grahams, or Cheerios. You could also add extra white chocolate chips or M&Ms candies.