Pop the bag of popcorn and pour the popped corn out onto a baking sheet lined with a silpat or parchment paper. Remove any unpopped kernels. Mix in nuts and marshmallows.

In a large microwave-safe bowl, microwave butter, sugar, and corn syrup, stirring every 30 seconds. When mixture comes to a full boil, mix in chocolate spread until well combined. Dollop chocolate mixture evenly over the popcorn. Using a spatula, toss the popcorn to coat it with the chocolate mixture.