Line a 9x9” pan w/foil and spray w/cooking spray. Place 16 p butter cups in an even layer on the bottom of the pan. Place choc chips and sweetened condensed milk in saucepan over low heat. Stir until chips and milk are melted. Spread over p. butter cups in pan. Immediately sprinkle with the crumbled Reese's, press lightly into fudge. Cool until firm, then cover with plastic and place in the refrigerator to harden completely.