The recipe is from a 1980 Pillsbury booklet, and I suspect the problem is that frosting is not made the same way now as it was then and that a different type of fat is being used. This may explain why the frosting did not firm up again quickly.
However, because everyone who has tried them has liked the flavor so well, I've decided to post the recipe anyway. Feel free to try them as is, or to coat them with a sturdier chocolate, such as melted chocolate chips or chocolate bark.
Peanut Butter Bon Bons
1 can ready-to-spread vanilla frosting
1 cup peanut butter
1/4 cup butter, softened
2 cups graham cracker crumbs
1 can any flavor of ready-to-spread frosting (for coating)
In large bowl, combine vanilla frosting, peanut butter and butter with fork until well blended. Stir in crumbs until well mixed; Form into 1-inch balls and chill for 1 hour.
In small saucepan, melt remaining frosting over low heat, stirring occasionally. Dip chilled balls in melted frosting; allow excess to drip off. Place on cooling rack over waxed or parchment paper (or a clean cookie sheet); let stand at room temperature until dry, about 6 hours.
Makes 48-52 bon bons.
Tip: As frosting in saucepan thickens, reheat. You can also scrape the dripped frosting from the waxed or parchment paper and return it to the pan to reheat it.