Spray a 9-by-13-inch baking dish with nonstick cooking spray. Set aside.
Melt the peanut butter and butter together in a medium saucepan over low heat, then stir in the marshmallows. Continue to stir until the marshmallows are melted.
Remove the pan from the heat and stir in the popcorn to coat evenly in the melted marshmallow mixture. Pour the popcorn mixture into the coated baking dish and spread out into an even layer. Drizzle the popcorn with the chocolate sauce. Allow the popcorn treats to cool for 30 minutes, then cut into bars.