Place cupcake liners in a muffin tin. In a microwave-safe bowl, combine 6 ounces of chocolate chips with 2 tablespoons of butter. Microwave in 15-second intervals, stirring after each interval, until melted and smooth.
Use a small pastry brush to coat the bottom and sides of each cupcake liner with melted chocolate. For full-size liners, brush the chocolate about one-third of the way up the sides. For mini liners, brush it about halfway up. Refrigerate for about 5 minutes, or until firm.
In a medium bowl, stir together the graham cracker crumbs and powdered sugar.
In a separate microwave-safe bowl, combine the peanut butter and 6 tablespoons of butter. Microwave in 15-second intervals, stirring between each interval, until melted and combined.
Pour over the graham cracker mixture and stir well.
Press approximately 1 tablespoon of peanut butter filling into each full-size cup or about 1 teaspoon into each mini cup. Flatten the tops and keep the filling below the chocolate-covered sides.
Add the remaining chocolate chips and remaining 2 tablespoons of butter to the bowl containing the melted chocolate. Microwave in 15-second intervals, stirring frequently, until smooth.
Spoon the chocolate over each cup until the filling is completely covered. Gently tap the pan on the counter to smooth the tops.
Refrigerate for about 20 minutes, or until the chocolate is firm and no longer shiny. Remove the cups from the liners and serve, or transfer them to an airtight container for storage.