Place the chocolate in a microwave-safe bowl and microwave it at 100 percent power 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 also melt the chocolate in a large bowl over a pan with 1 inch of barely simmering water, stirring the chocolate occasionally, until it is completely melted).
Using a small paintbrush or pastry brush, coat the bottoms and 3/4 of the way up the sides of the liners with some of the chocolate (set aside the rest for later). You want the coating to be fairly thick, so that when the liners are peeled off, the chocolate shell doesn't crack. Place the filled 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 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; about 10-15 minutes should do it (if the mixture is too hot, it will melt the chocolate shells).
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Remove the chocolate cups from the refrigerator. Use a teaspoon to fill each shell 3/4 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 has 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.