Place 16 paper liners in a mini muffin pan (if not using a silicone pan)
In a microwave-safe bowl, combine the chocolate chips and unsweetened chocolate. Microwave at 30-second intervals (stirring in between) until chocolate is completely melted.
Spoon chocolate into muffin pan (or liners if using). Use a small spoon to coat the sides of the cups with chocolate. Put the pan in the freezer to chill the chocolate.
In a small bowl, combine peanut butter and melted butter. Stir in confectioners sugar until smooth.
Distribute peanut butter mixture evenly atop the hardened chocolate and press gently.
If needed, reheat the chocolate for about 20 seconds. Spoon the remaining chocolate over the peanut butter. Tap the pan several times to even out the chocolate and flatten the cups. Put pan in refrigerator and chill until completely firm (at least 20 minutes).