Warning! These peanut butter cups aren't light or low-fat, just super delicious and decadent and worth every calorie. They're what I make when I need to give myself a real honest-to-goodness chocolate treat. This is my dad's favorite dessert by a mile.
Adapted from "Pretty Delicious" by Candice Kumai.
Place the chocolate in a microwaveable bowl and microwave in 10-second intervals, stirring between each, until the chocolate is melted. Set aside to cool slightly and then scrape it into a large bowl. (You can melt the chocolate in a large bowl over a pan with one inch of barely simmering water, stirring the chocolate occasionally, until it is completely melted.)
Using a small paint brush or pastry brush, coat the bottoms and three-fourths of the way up the sides of the cupcake liners with some of the chocolate (set aside the rest for later). The coating should be fairly thick so that when the liners are peeled off, the chocolate shell doesn't crack. Place the coated liners on a plate and refrigerate for at least 20 minutes or up to several hours.
While the shells chill, make the peanut butter filling. Melt the butter in a medium saucepan over medium heat. Stir in the peanut butter filling. Melt the butter in a medium saucepan over medium heat. Stir in the peanut butter using a wooden spoon and then add the confectioners' sugar and rice cereal. Mix until well combined and gooey looking (it should look like slightly melted chunky peanut butter). Allow the mixture to cool slightly; 10-15 minutes should do it (if the mixture is too hot, it will melt the chocolate shells).
Remove the chocolate cups from the refrigerator. Use a teaspoon to fill each shell three-fourths full with peanut butter filling (you can use a wet finger or the back of a spoon to smooth out the top if you like). If the remaining chocolate in the bowl had hardened, re-melt it in the microwave in 5-second bursts. Spoon some chocolate over the peanut butter filling, spreading and turning the shell so the chocolate seals the cup. Return to the refrigerator for at least 20 minutes (or up to 2 weeks in an airtight plastic container) to harden. Serve chilled.