Preheat oven to 200 degrees F. Line a large baking sheet with parchment. In a large bowl, combine popcorn, chips, and walnuts.
In a large sauce pan, combine the butter, brown sugar, salt, and corn syrup. Bring to a boil over medium high heat. Boil for 5 minutes, then remove from the heat. Add in baking soda, stirring well. Mixture will bubble up quite a bit. Pour over the popcorn mixture and stir to evenly coat. Dump onto prepared baking sheet. Bake for 40 minutes, stirring twice while it cooks. Remove from oven. Let cool slightly, then gently stir in marshmallows. Let cool completely, then serve.