Line the bottom and two opposite sides of an 8x8-inch baking pan (about 2 inches high) with a sheet of parchment paper so that it hangs about 2 inches over the sides of the pan to form a sling.

Cut 6 tablespoons unsalted butter into 6 pieces and place in a large pot. Melt over medium-low heat, gently stirring with a flexible spatula so it melts evenly. Turn off the heat. Add ½ cup creamy peanut butter, ½ teaspoon vanilla extract, and ¼ teaspoon kosher salt, and stir until combined and smooth.

Working quickly, add half 1 (10-ounce) package mini marshmallows, and stir until thoroughly melted. Add the remaining marshmallows and stir until melted. (Return the pot to low heat if the marshmallows aren’t melting easily, but keep a close eye on the butter, as you don’t want it to continue to brown to the point of burning.)

Add 5 ½ cups rice crisp cereal and gently fold it in until the cereal is completely coated with the marshmallow mixture. Transfer half of the mixture to the baking dish and gently press into an even layer, pushing it into the corners with the spatula or buttered hands. Arrange 16 full-size peanut butter cups in a single layer over the cereal mixture. Top with the remaining cereal mixture and gently press into an even layer.

Place 1 ½ cups semi-sweet chocolate chips in a medium microwave safe bowl and microwave in 30-second increments, stirring between each, until melted, 1 to 1 ½ minutes total. (Alternatively, melt on the stovetop over low heat.)

Pour over the bars and spread into an even layer. Refrigerate until the chocolate is set, about 40 minutes. Grasping the excess parchment, transfer the slab to a cutting board. Cut into 16 pieces.