Line a 24-cup mini muffin pan with mini paper liners. Set aside.
Place chocolate chips in a microwave-safe bowl. Microwave for 30 seconds; stir. Microwave for another 30 seconds; stir. Add 3 tablespoons of coconut oil and microwave for 30 more seconds. Stir until completely melted and blended. If chocolate chips are not completely melted, you may microwave in additional 15-second increments, stirring in between. (Alternatively, you may melt together the chocolate chips and coconut oil over a double boiler, stirring until well combined.)
Into each mini paper liner, spoon about 1 teaspoon of melted chocolate. Use the spoon to push chocolate up the sides of the paper liner, as high as possible. Freeze for five minutes or until chocolate is set.
While muffin pan is in freezer, melt remaining 3 tablespoons coconut oil. Stir oil into peanut butter until completely blended.
Remove muffin pan from freezer. Divide peanut butter mixture between chocolate shells (about 2 to 3 teaspoons peanut butter filling per cup).
If melted chocolate has started to thicken, microwave for 15 seconds and stir until smooth. Divide remaining melted chocolate between all of the peanut butter-filled muffin cups (about a teaspoon per cup). Use back of spoon to lightly spread chocolate towards edge of the wrapper, if necessary, until no peanut butter is visible.
Freeze muffin pan for 30 minutes. Remove from freezer and peel paper muffin liner off of each peanut butter cup. Serve peanut butter cups immediately or store in refrigerator until ready to serve.
You may use semi-sweet or milk chocolate chips, and they may be all-natural or regular.
If you use all-natural peanut butter, I recommend the "no-stir" kind, since it's typically less runny. Regular peanut butter will also work, if you prefer.
If you run out of enough melted chocolate to top the last few peanut butter cups, melt a few more chocolate chips with a small dollop of coconut oil in the mircrowave.
I find that the easiest way to peel paper liners from the frozen chocolate is to peel a strip down the side of each peanut butter cup, from top to bottom, and then peel the wrapper off sideways, all the way around the cup. From there, the bottom of the paper liner should peel off easily.