In a microwave-safe bowl, heat the butter and marshmallows for 3 minutes, stirring halfway through heating time, until completely melted. Add the peanut butter and stir again. Add 6 cups of crispy rice cereal and stir until well combined. Let cool slightly.
Place 1 to 2 tablespoons melted chocolate in the bottom of each liner. Form small puck-shaped pieces of peanut butter crispy treats with greased hands, and place in the chocolate in the liners. Top with more melted chocolate until the crispy-treat layer is completely covered. Sprinkle with sea salt and refrigerate until firm. Allow the cups to come to room temperature before serving.