Line standard 12-cup muffin tin with paper cupcake liners. If desired, trim the cupcake liners to shorten, but it's not necessary.
Gently melt the dark chocolate, unsweetened chocolate, and coconut oil over a double boiler over low heat.
Spoon one tablespoon of melted chocolate into each muffin tin cavity. Use the back of the spoon to gently push the melted chocolate up the sides of the paper liner. Place muffin tin the fridge until the chocolate sets, about 10 minutes.
Spoon ½ tablespoon of peanut butter evenly into the chocolate cups, making sure there are no air bubbles. Spoon another tablespoon of melted chocolate over peanut butter. Sprinkle cacao nibs on top, if desired. Place muffin tin back in the fridge for another 10 minutes until chocolate is set.
Transfer peanut butter cups to an airtight container. Keep at room temperature (if it's not warm) or in the fridge until ready to eat for up to a week.
NOTES:• It's recommended that you use natural peanut butter that only contains two ingredients: peanuts and salt.