In a large stockpot, melt peanut butter, butter and marshmallows over medium heat. Use a wooden spoon and a strong arm to stir in the crispy rice cereal, then transfer the mixture to a parchment-lined 8x8" baking dish and press it evenly into the bottom (it helps to use slightly dampened hands).
Warm the caramel sauce slightly in the microwave (microwave-safe bowl, please) or on the stovetop then spread it evenly over top of the crispy rice base layer. Transfer the pan to the freezer for an hour or until caramel is completely firm.
Meanwhile, melt the chocolate in the microwave at 50% power for 2 minutes until the chocolate is about 80% melted then stir vigourously to finish melting. (Continue in 30-second intervals on 50% power if needed, but be careful - you don't want to scorch the chocolate or it will seize).
Let chocolate cool for about 20 minutes before dolloping it over the caramel layer, then spreading it out evenly using an offset spatula. Let the treats stand at room temperature until the chocolate is firm, about 2 hours, before lifting them out by the parchment paper and slicing them into 16 squares. (You can speed things up by popping the pan back in the freezer, but you might get a chalky bloom on the chocolate - nothing problematic, it just won't be as glossy and attractive.)
Store the treats in an airtight container at room temperature for up to 3 days (if they last that long, which they won't).