Line an 8x8" pan with foil and spray with cooking spray. You can also use a 9x9" pan, but your fudge will be thinner than shown.
Place butterscotch and white chocolate chips and sweetened condensed milk in a medium saucepan over low heat. Stir constantly until melted and smooth.
Remove from heat and stir in vanilla, chopped Oreos, and 1/3 cup mini Reese's Pieces.
Pour into prepared pan, spread to fill and sprinkle with an additional handful (or two!) of the Reese's pieces, for garnish.
Let come to room temperature, then chill to set. (You can also just leave it out over night to set, if your house isn't too hot.) Slice into squares and serve.
Makes about 36-48 squares, depending on size.