Line an 8- or 9-inch square pan with aluminum foil or parchment paper and set aside.
Unwrap the Reese's peanut butter cups and chop them into pieces; set aside some pieces for pressing on top if desired.
In a medium saucepan over low to medium-low heat, combine the chocolate chips and sweetened condensed milk. Stir constantly until the chocolate is fully melted and the mixture is smooth, about 2–3 minutes.
Remove the pan from the heat and pour the chocolate mixture into the prepared pan, spreading it evenly with a spatula.
Press the chopped peanut butter cups onto the surface of the fudge so they adhere, then let the pan cool at room temperature briefly before refrigerating.
Chill the fudge until firm, at least 2 hours, then lift from the pan using the foil or parchment and cut into 32 pieces.