Beat the peanut butter, softened butter, and vanilla with a mixer on medium speed. Stop when the mixture looks lighter and fluffy, with no visible streaks of butter.

Add the powdered sugar gradually while mixing. The mixture should become thick and cohesive, with the sugar fully absorbed.

Fold in the crushed Butterfinger bars, saving some pieces for the tops, then refrigerate the mixture for 30 minutes. It should feel firmer and less tacky when you scoop it.

Roll the chilled mixture into balls on a parchment-lined baking sheet, then refrigerate them for another 30 minutes. The balls should hold their round shape when lifted.

Put the chocolate chips and oil in a small microwave-safe bowl, heat at 50% power for 1 minute, and stir. The chips may still look partly intact but should begin to soften as you stir.

Continue heating at 50% power in 30-second increments, stirring after each, until the chocolate melts. Stop when it turns smooth and flows easily from the fork.

Lift a chilled ball with a fork, coat it completely in chocolate, let the excess drip off, and return it to the parchment. The coating should cover the center without collecting in a thick puddle underneath.

Sprinkle the reserved Butterfinger pieces over the wet coating and repeat with the remaining balls. The crumbs should stick to the chocolate before it firms up.

Leave the coated balls at room temperature until the chocolate sets, then serve. The shell should feel firm to a light touch.