Combine the peanut butter and powdered sugar until the peanut butter is no longer sticky. You may need a little extra powdered sugar.
Roll the peanut butter into 12 balls, pressing each slightly flat to form patties. Set aside.
Line a mini muffin pan with 12 paper liners. (Greasing the pan does not work well for this, so it's really best to use paper liners here.)
Melt the chocolate in a double boiler over medium-low heat or in the microwave (see notes) until smooth.
Spoon a small amount of melted chocolate into the bottom of each paper liner.
Lightly press a peanut butter patty into each cup on top of the melted chocolate - the peanut butter should not touch the bottom of the cup.
Spoon melted chocolate over top of the peanut butter patty, making sure it gets on the sides of the patty as well, filling the cup to the top. Tap the pan on the counter to smooth out the top of the chocolate.
Let set for at least 30 minutes, or until chocolate has fully hardened, then enjoy!
Notes
You can create a double boiler by placing the chocolate in a heat safe bowl on a saucepan filled about ⅓ of the way with water. Ideally the bowl will not be in direct contact with the water. Stir the chocolate frequently.If melting the chocolate in a microwave, heat at 50% power for 30 seconds then stir, and continue heating for 15 seconds at a time stirring thoroughly after each one.Be sure not to get water into the chocolate or it may cause the chocolate to seize, making it unusable.These can be made into 6 full-size peanut butter cups using a standard muffin tin.For a softer chocolate shell around the peanut butter filling, add 1-2 tablespoons of peanut butter to the chocolate before melting, and allow the peanut butter to melt with the chocolate.