Place the butter and peanut butter into a medium bowl, and beat with an electric mixer (or hand mix)Gradually mix in the sugar, and when it starts to get thick, incorporate milk one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable. Beat for at least 3 minutes for it to get good and fluffy. Place on cake or cupcakes. Garnish with mini reeses and shaved chocolate.