Butter an 8-inch square baking pan. Line with 2 crisscrossed pieces of parchment, buttering in between and leaving and overhand on all sides; butter the parchment.
Melt the butter, marshmallows, salt, and 1/3 cup of the peanut butter in a medium pot over medium heat. Remove from the heat and stir in a rice cereal. Press into the prepared pan.
Place the chocolate and the remaining tablespoon peanut butter in a large microwave-safe bowl. Microwave on high in 30-second intervals, stirring in between, until melted. Spread on the bars. Sprinkle with the peanut butter cups. Let sit until the chocolate is set, 1 to 2 hours.
Holding the paper overhang, lift the bars out of the pan and transfer to a cutting board. Cut into 16 squares (4 rows by 4 rows).