Melt butter in a medium saucepan over medium heat. Stir in peanut butter and cinnamon (if desired) until butter is melted and mixture is blended. Stir marshmallows into mixture until melted and well blended.
Pour over popcorn and stir until evenly coated. With buttered hands, shape popcorn into about 18 small popcorn balls (1 1/2inches); place onto parchment lined baking sheet.
Flatten each ball slightly. With a serrated bread knife, carefully cut each cookie in half horizontally. Spoon about 1 teaspoon of hazelnut spread onto cut side of one half. Top with 2nd half; repeat with remaining popcorn balls.
Serve immediately or store in an airtight container.