Add the dark chocolate to a microwave-safe bowl and heat in 30-second bursts, stirring in between, until fully melted and smooth.
Pour the melted chocolate straight into the popcorn bag, seal or hold it closed, and gently shake and massage the bag until all the popcorn is evenly coated.
Flatten the mixture inside the bag using your hands or a rolling pin so it forms an even layer.
Place the bag in the freezer for 30 minutes, or until set, then remove, tear the bag open, and cut into bars.