How to Make It
Preheat oven to 250°F. Spread shiitakes in a single layer on a parchment paper-lined baking sheet. Bake in preheated oven until dry and brittle, about 1 hour and 30 minutes.
Process shiitakes, salt, Parmesan, and rosemary in a food processor until finely ground, about 1 minute. Sprinkle on buttered popcorn.