For a milder and sweeter chocolate coating, try using milk chocolate or white chocolate instead of semi-sweet. Crunchy or homemade/natural peanut butter may be used as well. You could also make 24 to 30 smaller peanut butter cups in a mini-muffin pan.
Line a 12-cup muffin pan with cupcake liners. Set aside.
Using a hand mixer or stand mixer fitted with a paddle attachment, cream the peanut butter and butter together in a medium bowl on medium speed for 1 minute. Add the confectioners’ sugar and brown sugar. Beat until combined and smooth. Place in the refrigerator until ready to use.
Melt the chocolate and shortening together in a medium microwave-safe bowl in 20-second increments until melted, stirring well after each increment. Spoon 2 teaspoons of the melted chocolate into each of the cupcake liners. To allow the chocolate to settle evenly inside, bang the muffin pan against the counter a few times. Spoon 2 heaping teaspoons of the peanut butter mixture on top of the chocolate. Bang the pan on the counter or spread with a knife. If you want a traditional-looking peanut butter cup where the peanut butter layer isn’t exposed on the sides, don’t allow the peanut butter to reach the edges of the cup.
Spoon the remaining chocolate evenly on top of the peanut butter. Once again, bang the pan on the counter until the tops are smooth. Chill in the refrigerator for at least 1 hour. Cups stay fresh stored in an airtight container in the refrigerator for up to 7 days.