1/3cupplus 3 tablespoons mini Reese's Pieces, divided
In a large bowl using a handheld electric mixer, or in the body of a stand mixer fitted with the whisk attachment, combine the powdered sugar, peanut butter, vanilla and softened butter. Beat on low-speed until combined and slightly crumbly. Add milk, one tablespoon at a time, and continue to beat until the mixture becomes a workable dough. Fold in 1/3 cup of the Reese's pieces. The mixture should be a little sticky, but scoop-able and easy to work with.
Scoop 1 tablespoon-sized rounds of filling out onto a large baking sheet lined with parchment paper. Roll each round into ball. Place the baking sheet in the freezer to firm up for one hour.
During the last few minutes of the chill time, make the chocolate coating. You will make the chocolate coating in the microwave. Place the chocolate and oil in a heat-proof glass measuring cup and melt, on low-power, in 30 second increments, stirring after each increment, until completely melted. Whisk chocolate smooth and let it slightly cool before dipping.
Remove peanut butter balls from freezer. One at a time, carefully dip each chilled ball in the melted chocolate. When lifting the peanut butter ball out of the chocolate remember to allow excess chocolate to drip back into the bowl. Transfer the chocolate covered balls back to parchment paper lined baking sheet after dipping each one. Sprinkle the tops of the wet chocolate with remaining Reese's pieces.
Transfer the peanut butter balls to the refrigerator to set for 1 hour. Serve chilled.
1) If you like, roughly chop the remaining 3 tablespoons of mini Reese's pieces before sprinkling them on the wet chocolate. 2)For easy dipping use a small fork or specialty truffle dipping skewer. 3)If you don't own a microwave, you can make the chocolate coating in a double broiler.