Line a 9x13-inch baking dish with parchment paper, ensuring it extends up the sides for easy removal.
In a large mixing bowl, combine the popped popcorn, Rice Krispies cereal, and ¾ cup of the toffee bits. Gently toss until evenly combined.
In a medium saucepan over low heat, melt the butter. Once the butter has melted, add the mini marshmallows. Stir continuously with a rubber spatula until the marshmallows are completely melted and smooth.
Remove the saucepan from the heat. Stir in the vanilla and salt.
To a microwave-safe bowl, add the semi-sweet chocolate chips. Microwave in 30-second intervals, stirring between each, until melted and smooth.
Immediately pour the melted marshmallow mixture over the popcorn mixture. Lightly grease your spatula (or hands) with butter or cooking spray, then gently stir until everything is well coated. (Note: If using your hands to mix, let the mixture cool for about a minute first, as the marshmallows will be hot right after melting.)
Drizzle the melted chocolate over the coated popcorn mixture. Stir again until everything is fully combined and evenly coated.
Transfer the mixture to the prepared baking dish. Use a greased spatula to press it down firmly and evenly into the pan.
Sprinkle the remaining ¼ cup of toffee bits over the top and gently press them in.
Let the bars set for 10 minutes at room temperature.
Once set, lift the bars out of the pan using the parchment overhang. Slice into squares and serve.